^This. Social anxiety sucks so hard and it makes meeting people really difficult. If you aren't aware of someone having social anxiety, I can easily see how their behavior would appear as rudeness.

I agree with lyndis though, I've been raised that, generally speaking, if someone invites me somewhere, they pay for it. Conversely, if I invite someone to do something, I pay. The OP didn't address who invited whom though, so who knows what went down.