Yes, you are correct about that. Robert is an elephant in the room during that conversation. It would make sense for Ian to mention her seeing him at least in the situation where Lena accuses Ian not only for messing with Holly's feelings, but also because she feels hurt by Ian's "cheating" (lena_ian_love dialogue). Even if she broke up with Robert afterward, Ian doesn't know that yet. If she did break up with Robert, then I perfectly understand Lena's frustration, because she was willing to ditch her boy-toy for Ian, but then he turned out to be just like any other guy (if he in fact slept with Holly). But I totally understand Lena's reaction if she's not in love with Ian or if Holly kissed Ian. Holly is a gentle soul, in love with Ian, and Lena doesn't want to be the one hurting her, even indirectly so she chooses to "step down", at least until Ian and Holly figure it out between themselves, just like Cherry does when she learns about Alison's feelings for Ian. But I don't understand such a massive drop in their relationship regardless of their status, implying that there's more to that than her protective feelings for Holly.