I didn't say it would magically solve itself, in fact I said it would make it easier for him to get an artist to do portraits if he did what I suggested. Finishing a game is hard, and adding in this hurdle right now doesn't make it any easier.
If he keeps moving forward, the worst case scenario is he takes out the full-size character portraits -- and the game still works fine! Best case is he says, hey, my game is done, I just need new portraits, can anybody help? He can even REALISTICALLY offer a rev share at that point if he wants, as opposed to asking for it for a game the artist is not sure is even going to be finished.
Otherwise he gets stuck in the mud dealing with this, with all sorts of bad possibilities: he starts dealing with flaky artist after flaky artist, costing him time and dealing with incomplete work, or he has to go back and change things in his game to get a co-designing artist on board, which potentially robs him of his own drive to work on the project. Maybe the new art isn't all that great and now he feels obligated to put "bad" art in his game because the artist did it for free and they agreed on it. All sorts of bullshit to have to deal with.
I say just move forward, make the game, and deal with this during the polish stage because it's a polish-stage feature.
Well, you're assuming all he needs an artist for is character portraits, and is happy to use stock sprites for everything else. If that's the case then I would simply eschew them altogether indeed and worry about them later, if at all. I guess some clarification here would get us all in the same page.