I have a rule when it comes to big decisions like killing off a MC.
I have to answer tos questions:
1-Does this make the story better?
2-Why?
If the answer is no, or if you don`t have a plan for what happens next, it will probably be a terrible idea.
If you kill your MC, you have to make sure you have a good plan to move the story.
If he did some foreshadowing, or made some build-up to this moment it can help a lot. Either by making the death make logical sense or by making the moment of the death more meaningfull.(ex: the hero have sustained many wounds in a short period fighting a vampire king , but instead of resting, he fought a last time.He knew it would mean death to fight to the limits of his body, but nobody else could take down the vampire/dragon the vampire king owned that is treatening the kingdom)
This can be a chance to pass the torch to another team member or for a future generation. But it depends of a solid execution and great storytelling. (this needs a solid supporting cast to work)
As you said, killing your MC is a risky move, but if your friend is a good writer with a solid plan, he can make the story better with this.