IIRC Ian doesn't know who Gillian's boyfriend is. Billy is a very unlikely candidate, though -- Gillian's boyfriend moved to town a few months ago because he's found a job here, and that doesn't really fit Billy's circumstances.
edit: a fun fact, now that i thnk about it. When Chapter 10 was released, i thought the way the game avoided ever having Ian and Mike interact has gone to ridiculous level, with Jeremy (who is familiar with Mike, his coworker) for some reason choosing not to introduce him to Ian, his friend even though all three of them were in the same spot and shared the screen. But now, given Jeremy's reluctance to tell Ian about Gillian, his behavior regarding Mike would actually make certain sense, if Jeremy has learned recently that Mike is Gillian's new boyfriend, after seeing Gillian with him together at the club.
Guess we'll see.