You are too nice!! So now YOU have to ask her for coffee and then casually say to your boy, oh, chum, want to join us? Sounds awful!
Maybe get a handful of office mates to go for a beer after work (is that realistic?) and then just have one beer and skedaddle.
I know! I don't know what came over me! I think the beers after work idea is best. Now I just have to do the actual organising blegh