I said in another comment elsewhere, but where I live, if someone invites you somewhere, they're the ones who pay for everything. It can be seen as insulting if you offer to help with a bill--you're insinuating that they're poor hosts. No way to know if that's what the OP's friend had in mind or if they were just being a cheap jerk though.

That being said, yeah, I agree that every effort should be made to be polite to your host no matter what.