In this case, how do you know the act isn't phony? If you know that person is sometimes phony, you'll be left wondering whether anything they do for you is insincere, including buying lunch. I couldn't accept it if I smell something off about their intentions.
I think there are worse things than a lunch paid for someone who is insincere (such as having to pay for lunch with someone who is insincere). It's not really worth worrying about to me actually.
