My paper got accepted; acquaintance's paper wasn't. Should I thank them?
My paper was accepted, but I heard that their paper was not. Should I thank my acquaintance? On the one hand it is courteous to do so, since they read a draft and helped me to improve. On the other hand, it could come across as schadenfreude if they are a bit on the sensitive side.


Yes, please thank them. That your paper got accepted and theirs was not may or may not sting a bit, but that will be true independently of whether you thank them or not.

That being said, the protocol in such cases seems to be that they will approach you and congratulate you on your accepted paper, and this is a great time to heartfully thank them for their help and express that you are sorry about their paper being rejected.