Fml I'm in the same situation with a girl. It doesn't matter who else I get off with she just sticks about in my head. Stopped speaking to her months ago, haven't seen her in even longer but still after I've got with another girl it's like an instant 'wish it was her instead' afterwards.
I know it's ridiculous but have no idea how to deal with it. Am just gonna ask her out at some point seeing as this isn't going away. Worst bit it is I'm sure she was interested too but the timing was a little off.
Ahh well, just ask him out that way at least you know whether it's happening or not