I think if it suits the story, it is alright to kill the main character. And of course at the end of the story. (Except if he/she has more than one main character. Like in Game of Thrones.)
Some reader would be probably upset about it, but this is his story, so first he should decide what matters him more: the love of the readers or to write the story as he imagined.
But what if the main character was already dead?! Oooooh~ 
In all seriousness though, I don't think I'd kill off a main character of mine within the story (I love them too dang much 
). But I can see it working in others. Depending on what the story's about, of course, like others have mentioned. The limited comic series 'Daytripper,' for instance, is all about the myriad of ways the MC could have died at any point in his lifetime, and it is beautifully told. So, if your friend feels strongly about this then, well, it's his story to tell.
Not in my current story, but maybe in another and only at the end if the story calls for it. I feel like it's jarring to the audience if the main is killed off mid story somewhere unless there are multiple lead characters to allow another to pick up the slack. That's my opinion at least.
I could easily see a scenario where my main character's done everything he needs to accomplish in the story, but I like the side-characters and world enough that I want to stick in it a while and follow them for a bit. Killing of the Main Character or just putting them on a bus isn't the worst way to do this.
I would absolutely kill off my MC if the story needed me to or if the story leads to that. As an MC you have several layers of plot armor so you kind of know they are safe from death at least, unless you give them a hero's death or something like that.
However if I were to kill my MC I would start to shift the narrative to another character WAY BEFORE the death of MC, maybe several issues before, maybe add some death flags here and there, so when that moment happens the other character can take the mantle of MC and to kind of prepare the reader for that moment.
Well my current story's main is sort of in between life and death, sooo, this very awkward.
But to seriously answer the question. Yes.
If I'm were trying to make a point with their death then of course. Another lead can pick up where the main left off just not make their death less pointless.
I read a tip before that says do not kill a character just for the heck of it or to create a drama. Kill them if it really need to be done, if their death serves an important purpose for the plot.
Nope. My story is about jazz and the pursuit of fame- not the darkest thing out there. Characters will die on the inside a lot but never on the outside. :^)
I've alsways been the one that could easily kill off a MC.
BUT, that doesn't mean I did it every time. A charater shouldn't die to shock, or sadden the viewer. Heck, he shouldn't die to make things epic!
He only has to die, when his death is necessary. When it advances the plot, and progresses it an away that couldn;t be possible without his death 
If it fits the story, then go ahead! My main story right now doesn't involve killing a main character, but another project I have has a high probability of this happening. This story revovles around two main characters though, so you don't completely lose out on the point of view.
That's the tricky part of killing your main I think. You lose the character you see through the eyes of, so who do they follow afterwards? Are there other main players that can take over? have another perspective been introduced so it feels like a transition and not happens abruptly? Basically how does the story continue (or end if that's the point)?
My MC, for the most part, a big no. Side characters however...that could fund the coffin business for a while with some of my works (RIP secondary characters, you did not die in vain, but for the sake of the plot). The only time I have killed MCs was because it was technically a prologue that showed both in their previous life to explain how they got into the positions they are in now.
And there is no way I can with two stories I have because one is a slice of life, so...yeah. And the other has them being immortal and prize fought over by deities, so killing them through deus ex machina would have them lost to another god.
Granted, I do love to keep MCs alive to suffer much long in horrible ways, so that might be worse than if I was kind and just killed them. 
Definitely. Maybe it's because I write a lot of slice-of-life, but I approach the matter with the mentality of 'death is a part of life, and being a main character doesn't make them immune'.
Right now I'm working on an large story that will span four books and has twelve main characters (three per book). By the end of the fourth book, two of those characters will be dead. Both for a good, plot relevant reason; self sacrifice to save others.
At the same time, I get not wanting to kill off a main character because your readers like them. Heck, I grew really attached to a character I wanted to kill off and actually went looking for a way to bring them back. I needed to have someone tell me that bringing them back would cheapen the sacrifice they made. Now it's become my mission to make readers cry when the death eventually happens.
These answers really really depend on the type of story being told. Look at Shakespeare. I cannot think of one protagonist from any tragedy who survives to the end. I also cannot think of any protagonist in a comedy who dies. The bard was a little formulaic in that way.
In most stories that involve death and tension, the possibility of death for any character is almost certainly required to keep the reader engaged. If none of the MCs ever fail nor die, there is no tension left.
Well, let me think...
.
.
My comic's doesn't really need that sort of thing in it.
I would kill off the main character of my story if it is the direction that I want the story to go in. Sometimes this can make the story more realistic. If a non-fiction story shows events of a time period of two hundred years, killing the main character or having them die of natural causes makes the story more realistic. Now if someone wants to kill off their main character but doesn't do it to keep people reading for views, money, etc., might lose creativity if they are going against their instincts. I would keep viewing the story. If the story starts to get uninteresting, I might stop viewing it. I would still give the story a chance after the main character is killed or dies of natural causes. The story might even get better. Now if a story is created first and then the prologue is created second. If the prologue is the first thing shown to people, the main character might not even be involved yet.
