I can't remember exactly how long Mike has been in town, or if that was ever revealed in some convo with Lena. So maybe it fits Gillian's timeline, or maybe not.I will insist without being certain ofc, that Mike is Gillian's bf. And he will probably be Ian's "enemy" down the path, who's gonna win Lena over. Already those 2 are antagonists for Lena's horny calls. If you go for the cuck path it's probably Mike fucking another Ian's gf. If you go for the stud path it's who's the alpha, a big fight between Jeremy, Axel, Mike, Ian and Seymour down the line.
But we know Jeremy tells Ian that she was just with "some guy." If it was Mike, Jeremy would recognize him for sure because they work at the same club. And this dialogue isn't based on any variables, so Jeremy has no discernible reason to lie to Ian on a best buddies route.