This! Nothing pisses me off more than people who use their mental illness as an excuse to treat others badly. All it does is perpetuate the stereotype that people with mental illness are assholes, and gives assholes leeway to treat others badly because "they can't help it". Bull.

No mental illness prevents people from saying thanks or offering to pay for your own things. But a selfish personality sure does. The fact that the OP's friend didn't say thanks (or apologize) online once they got home shows that it had nothing to do with social anxiety, and everything to do with being an ass.