I just very recently told my gf about my fantasies and she for now is not into having sex with other men but she does enjoy showing off her boobs and receiving male validation and guys flirting with her.

There is a guy that once accidentally had seen her boobs when she was sitting on my bed with her bare boobs, my window was open (I live on the ground floor) and suddenly she saw a guy outside who saw her also. I closed the window then and smiled at him. Next time we saw him at that spot again (because his bike is chained there) and he was taking a glimpse to my window's direction.

Today I talked to him when we by chance met in the kitchen that we all use. It is a student dorm. I made small talk and he seems genuinly interested in building a connection (not sure if he remembers me). I saw on his insta though that he is in a happy relationship. (his gf lives in another country tho, they see each other once a month and he lives here - in the building that I also live in - for a year only).

in my fantasy I would invite him to my place while my gf would sit there with very revealing clothes and we would just talk and maybe play games or watch sth together and we would possibly enjoy him taking glances at her. if she ever wants to step up the game she could ask for a foot massage (and who knows if he would be okay with it). I also imagine how she someday wears lingerie and a short skirt that would basically partly reveal her pussy to him.

Is that morally right though? Should I leave him alone or is it in the end up to him?

I would just be worried a bit if he talks to other guys about me and my gf, I don't want her and my reputation to be dammaged in this little town. The people I know look down on relationships like this and I can't blame them. Not too long ago I was also doing that.