Hiya, I've just started dipping my toes into professional freelance illustration, and I have aspirations of starting my own studio someday.
(also sorry this post is going to be really long)
Theres this thing that Neil Gaiman said once, and it's that there are three qualities needed in order to make it as a professional.
- You can make outstanding work. Are you able to reach a level of quality that your peers cant meet?
- You can make deadlines. And can you make them reliably?
- You're great to work with. Are you nice or do you make the whole process of working with other human beings easier?
The other thing he said is that you only need two of those qualities.
- If you make good work and you're nice, nobody cares if you're no time.
- If you're on time and nice you're so great to work with that it's okay if your work isnt great.
- If you make good work and you're on time, nobody can complain that you're not nice.
Ideally someday you hit all three qualities, but nobody is perfect.
Personally I make good work and I'm nice to work with, I'm not great with making deadlines (I try my best though!). This works for me because since i'm fairly new to freelancing I dont have any sort of reuptation, so I have to rely on first impressions, and being nice and having good work makes a very good first impression.
If your friend meets two of those qualities they'll be fine I think? I would argue that they also need to be able to produce a consistent quality of work like @Kaykedrawsthings said.
If I were to hire an artist for something I would look for three things
1) Does the work they produce fit what i'm trying to do (that's super subjective so yeah, and it depends on what the standards are for the thing I'm trying to do.)
2) Are they able to take feedback and fix things if its pointed out to them.
3) Can they take my vision for something and develop it into something better.
Lets say i'm working on a cutsey mobile game, if your friend comes to me with a portfolio with paintings on par with the work of Normon Rockwell, I probably wont hire them, even if I really like their work, because they havent shown me what I'm looking for. If they come to me with a sort of sub-par cartoon style, I'm more likely to hire them, even if it's not anywhere near the same level of quality as the Normon-Rockwell-Esque portfolio.
From there I might point out something about a piece in their portfolio and see how they take it "I liked this piece but I thought the colors were a little bit weird." - if they respond to that really defensively and try and justify their decisions, I probably wont hire them because they showed me that they're not flexible and that they cant take feedback. If they say "Oh yeah I think you're right, perhaps it would have been better if I used an analogous color pallete or a more saturated green" then I'm more likely to hire them, because they showed me that they can take crituiqe and come up with multiple solutions to a problem.
After that I would tell them what I have in mind for them to do, if they nod their head and tell them that its the best thing they've ever heard and that it's perfect the way it is, then I wont hire them, because I dont need them to make it better since they already think its perfect. If they take my idea and come up with ways they could work with it and they further develop it, then yeah I'll probably hire them because they just showed me that they can take my project into new and better directions.
TLDR it's not nescessarily about the quality of your work, but more about how easy you are to work with.
EDIT: I thought of one other thing I wanted to add. - "Profesional quality" is relative to whatever type of work you're looking for. For example, If I wanted to do illustration for card games my work needs to be on par with the quality that you would see in a card game. If I wanted to do illustration for baby books, my work needs to be on par with other baby books. Theres not really a magic "profesional quality" that encompases all lines of work, nor should there be.