i would say no- on the basis that it is in their possession and removing it now would be a bit shady/awful. they obtained it rightfully in the time and it shouldn't be revoked now as a punishment. i know it's painful to fall out with good friends, but it's better to ignore and move on in this case.
I have someone blocked who used to be my good friend, and I've drawn art of their characters, but even though they're blocked, the art I drew for them is still up. Would there be any way to revoke the art or something?
There's no way to revoke the art, but if you're credited as the artist, you can open your "Art" tab on your TH profile and click that little checked box in the bottom right corner of every registered art piece you don't want displayed and change the credited username to something else, like a throwaway account that you'll never look at again. That way you still retain proper credits, but you don't have to deal with seeing it on your main account feed.
Could you: Possibly, but it'd really come down to the odd rules of DMCA laws, So not sure if you could because it constitutes as a gift or not.
Should you: Probably not. This is a frowned upon practice, while you have all the rights to your art (Uhh besides their character haha), it can come off as a little petty doing this. I'd say just let them keep the art unless something really, really bad went down between you two or they start using it inappropriately.
Best advice just let them keep it up.