I can't speak for your friend, but it sounds like hardcore social anxiety.

I've been in that position before (still kind of am), and it sucks. Because people will assume you're rude or just weird, regardless.

Having come from that, yes, sometimes a simple "Thank you" is really hard to do. Social interaction terrifies the socially anxious, and even a simple "thank you" can be a major slip up that you don't dare to take at the risk of fucking something up. It feels like a confrontation. Because the wrong tone, the wrong anything, can drive people away.
Hell, even freaking out and then having to explain that you're really anxious can drive people away. It's not an easy thing to deal with when our society has an expectation for absolutely everyone to handle social situations with calm and grace, or else they're "rude" or a "creepy weirdo."
Been there. Done that. Not fun.

I would maybe ask your friend about it, over the internet. Mental conditions like that aren't really something you can just go "hurr hurr get over it." Otherwise, you become the dick.


That being said, it was pretty dickish of them not to offer to pay anything.