Why would a FWB invite her partner to Thanksgiving?

Honestly? I think it's rather a personality thing, sometimes people are very outgoing and see that you'll be alone on a 'family gathering' and they'll want to do something nice and it would sadden them to see you alone on Thanksgiving. My mother is like that, she basically invites a lot of people on Christmas who are alone, even if it's supposed to be a 'family thing'. I guess the girl might be the same kind of person my mom is. Or she could like him more than just a friend, but honestly I wouldn't read to much into it.