I don't doubt that Jeremy really cares for Ian and wants the best for him when he tells him he shouldn't fall for Lena. But the fact is Ian does tell him that he's falling for her. If Jeremy still doesn't see Lena as Ian's love interest, that's Jeremy's mistake. As his friend, Jeremy can obviously give Ian his advice, but he doesn't have the right to put his opinion above Ian's feelings. If Ian tells Jeremy he's got feelings for Lena, that's the way Jeremy should treat her as well.
But as long as Jeremy's relationship to Ian is OK, he does tell Lena about Ian's feelings before the blowjob. And at that point it's also obvious to Lena that her connection to Ian is deeper than to Robert, for example (depending on your playthrough, of course). Lena brushes it off because she's drunk and horny, and seems to rationalise it as something harmless, but with what she knows of her own relationship to Ian, Ian's feelings for her, and his relationship to Jeremy, she should know that what she's doing is different than some other random hook-up.
All of that is true, but I wasn't really excusing Jeremy's actions. I was merely arguing from Ian's POV who likely to perceive events differently from you and me due to the lack of data necessary to reach to the same conclusions. Ian didn't openly confess his feelings to Lena at that point, and so has no reason to know that she suspected the truth or that Jeremy even told her that. So, based on that mindset, Ian is likely to forgive Lena in that scenario because she was officially single, and like you said, was drunk and horny in the company of her friends, and he never told Lena that Jeremy is off-limits, can't blame her for wanting his D (considering his "girlfriend" was okay with it). It might be just me, but Ian doesn't seem to have a lot of ground to stand on here to confidently confront Lena about it and risk pushing her away.
Ian also knows Jeremy is a horndog, and is unlikely to expect of him to act in a holy way in a very sexy scenario. Obviously he can't say no to a model-looking girl willing to suck his dick, with approval of his girlfriend to add to the excitement. If I was Ian, learning that Jeremy actually tried to resist despite all that would come as surprise to me. Based on interactions in Alison's path, Ian doesn't like Jeremy's twisted definition of the bro-code, but can't really change his mind about it so he begrudgingly accepts it. "Doesn't count unless she's your girlfriend". Morality of Jeremy's action is kinda irrelevant here, it's just whether it's too much for Ian to tolerate as something of"Jeremy never changes" variety to the point of being willing to break his friendship with Jeremy over it, or he is able to understand the situation, however weird it might sound, and realize that in Jeremy's shoes, he wouldn't really be better in a similar situation.
For the sake of honesty, let's try to flip the scenario and imagine that Jeremy had serious feelings for Ivy. And let's imagine it was Ian at that party instead of Jeremy, and Ivy volunteered to suck him off (with Lena's agreement and encouragement). Would Ian really be thinking about Jeremy's feelings at that moment, especially seeing that Ivy doesn't really think about Jeremy either? Or would Lena be his primary worry, and seeing that she's there with him, also horny, and for some reason willing for him to experience her friend in that way make him completely not care about Jeremy's existence at that moment? To what extent would he resist Ivy's advances? Would he find strength within to say "begone, thot!" and think of his bro's feelings or would Ivy and Lena's "pretty, please" drain the blood from his brain into his cock? I can almost guarantee that it would be a no-brainer decision for the absolute majority of the players here. And so, can Ian really blame Jeremy here if he with 80% certainty would break his regular bro code?
I totally imagine Ian would be pissed at the situation itself and be very uncomfortable with the fact that it happened, but it's one of those things where nobody is really to blame and hold accountable here to the point of burning bridges. If anything, Ian could agree to Jeremy's next sparring session to get it out of his system, and feel better after kicking his ass. And Jeremy would probably suggest it even and take one for the team.