Lena doesn't tell anyone about her feelings for Mike. As far as Ivy is concerned, Lena is just hooking up with the guy and couldn't care less if he fucks someone else. He already does his girlfriend after all.Doesn't Ivy contradict this by sleeping with Mike if Lena is in love with him?
This is a bit different from potential scenario where Lena fucks Jeremy even if the latter is aware, by Ian's own admission, that she's someone Ian has feelings for. Ian will actually mention it during their Ch.12 scene where Jeremy tells Ian about Lena's actions.