A quick follow-up, because i was replaying the last update, and turns out i forgot about this bit:It just that when people take emotionally massive hit they tend to indirectly approach those things. Some people I watched around show this exact behavior referring to the person the one their partner cheated with as "that one" etc. Even Gillian mention him as Her boyfriend and not mentions his name(check how she talks about him).

I think this is rather conclusive, that Ian doesn't actually know the identity of Gillian's new boyfriend. I mean, you don't refer to someone as "whoever he is" if you actually know who they are. Even if you'd rather refer to them as "that person".