I'm posting this to inquire for my brother. I've personally never have had much experience with single speed bikes and my brother recently got an old peugot 9 speed bike and he took the gears off to make a single speed. He got the fixed wheel rear tire but the geared crankset stayed. The chain stays on fine but after a few mins or riding, it comes off everytime. We've tried tightening the chain, loosening it up but it falls off everytime. He's getting frustrated because he's had it for a few months and can't seem to fix it. Would the solution be to get a single speed crankset or is there something else we can try?

Also, he has the single speed chain he bought as well.

Thanks