Will you go for a trip with your ex-gf (only two of you)?
And you know she still has feelings for you?
The situation is: she wants to go for a trip, but her friend cannot go at (almost) the last min; the city she is going is the place you always want to go, will you go with her?
If the guy decided to go with the ex-gf, what does it mean? Or, what is he thinking about?
He knows there is no "friends with benefits" type of thing in her world.