No, that's not correct. While she is not in relationship with Ian Lena can override Jeremy's reluctance to let her blow him. She is convincing him by telling him it's just a game and it's for the sake of his girlfriend. Afterwards, she can sext Jeremy and further convince him that he shouldn't be concerned about Ian because she is not, and that he should keep it secret. That's the point Jeremy stops arguing, after Lena basically seduces him throwing herself at him. Then she can visit him in the morning to blow and/or fuck him. If she does, she does not get into relationship with Ian, and eventually has a threesome with Louise and Jeremy.
It's not NTR because at no point of this Lena is in actual relationship with Ian. She simply decides she prefers Jeremy and follows through. That hardly makes Jeremy a "shit friend", it's not him who made all moves, but Lena (i.e. the player) Ian might have feelings for her, but Lena evidently doesn't, not enough to want a relationship with Ian, at least.
edit: amusingly, if down this route Lena can enter actual relationship with Jeremy, it might potentially lead to Ian being "a shit friend" by fucking Lena behind Jeremy's back. /s