But they did say they weren't bored. They said they were "fine"...they nodded, etc.

I'm also an introvert, I have been at gatherings where everybody else is having fun and talking and socializing and whatever. I'm the one on the side watching and listening. I'm actually not having a bad time. People will come to me and ask me a million times if I'm "okay" or if I'm enjoying myself and I will assuring them that I'm "okay" or I'm "fine" and I really am. Maybe she's just not very effusive in real life.

(I can also understand anxiety over the check as someone up above mentioned about "missing the window". I have similar thoughts when it comes check time if it wasn't already brought up. That said, I have managed to learn to ask up front how we're going to be paying/splitting. So this person being talked about may have to learn how to do that. Or maybe they really are just a mooch, I don't know. If it's truly damaging the friendship (and I can understand why an experience like this could) then the OP needs to either get it out there and explain what is bothering them or just tactfully back off the friendship.

And remember in the future to make clear they plan on splitting the bill.)