True, however, the player not only makes decisions for Lena, but also for Ian. Which means there has to be agency for him to figure out that something is fishy going on between Lena and Jeremy, and that means his friendship with Jeremy would have to be tested. If Jeremy prioritises keeping Lena's secret (player's decision) over being a loyal friend to Ian (player's investment to keep the relationship high and desire to play Ian like a smart guy), at some point Jeremy would have to favor one player-controlled decision over another which negates the player factor from this equation, and only leaves Jeremy's priorities as they are.
I was about to expand on this, but it would have got overly long. Basically, it would be a choice between player agency and immersion (for Ian's side) vs preventing quite a convoluted branch that will affect a lot of the game. That said, the player is playing both sides, so if you go for the BBC, you probably wanted that cheating, morality reprehensible relationship. If you want Ian to come out on top and be a smart, confident, successful dude, you should not have made Lena do that. That is the nature of crafting your own story within the framework of the game. However, there will certainly be people who want Lena to bang Jeremy, but also want Ian to be a smart confident guy who won't be oblivious. It does need to be addressed, right?
There is just no 2 ways about it. If you give a choice in the matter to Ian, it WILL create a route where he finds out. That will lead to shattered friendship and broken up couple. This will affect a huge chunk of the game. I'm just saying that this part of the equation could be as delayed as possible so that the story can more or less go on without much adjustment. The players will get the Lena x Jeremy that they opted in for without leading to massive changes in the character relationships. It could just be addressed by conditional flavor text. The player could make a series of decisions as Ian throughout, I dunno, 6 chapters or whatever. It could be ian_jeremy_suspect or something. If you reached the max by the end, the reckoning happens. If you reached something in the middle, he might be a little suspicious but choose to believe. If you get low or no points, he could be oblivious.
I just don't think a direct confrontation about the BJ/sexting issue (and subsequent cheating with Lena when she falls for the BBC temptation) too early is a good idea for MANY reasons, both narratively and in terms of game design. Ian just has to take one for the team here. He doesn't have to appear stupid for not realizing if Eva doesn't write incredibly obvious situations like Lena and Jeremy banging on the couch next to Ian as they watch football (real scenario in some porn lol).