Sure, she ignores personal boundaries and she's manipulative. But I don't see her telling Mike (if she did) about Lena's affairs in order to make herself look as the better girl in his eyes. I just don't see Ivy giving a fuck about Mike's opinion on her or making an effort to improve her odds to have sex with him again at the cost of Lena's. I see it more likely her just being gossipy or talking about Lena's exploits in a "don't get cocky kid, my Lena gets around, you're not special" way.Sure, but we're not talking about why Ivy fucked Mike, We're talking about why Ivy (if true) told Mike about Lena's personal affairs.
You may ask why i suspect Ivy in the first place ?
The reason is this isn't the the first time Ivy does this, Remember how she told Lena about Holly and Mark ? and she even showed her a picture of them doing it and she didn't even ask Holly if it's okay to show it to Lena or even tell her about it.
It should have been Holly's decision whether or not to tell Lena about Mark not Ivy's... and some people here think that Ivy is "helping" Holly to break out of her shell but the truth is she's just a puppet that Ivy is playing with.
But lacking Ivy's pov about her real motivations, who knows?, we can just guess.