It's a free game, I think people expect ads by now. Maybe not every time the game starts up (or having a daily limit) but that's not the worst thing in the world.
The options are:
1) a bunch of weird ads
2) a single IAP labeled "give me some money" that does nothing but gives the dev some small amount of cash.
3) a series of IAPs labeled "give me cash", "give me more cash", "give me even more cash", "give me loads of cash", which are set at increasing amounts. These again do nothing except give the dev some cash.
If he wants to monetize it he really better change the graphics first (though technically he's still copyright infringing regardless of whether he is charging for his game).