I think the issue is that the author really fumbled her introduction. As the MC himself says, everyone has their public face that they use to hide their private self. In general the character itself is fine and could be really interesting but I don't think we were given enough to either sympathize with her or at least see the drastic change as realistic. Obviously people can act really different in different parts of their lives but generally the same trends carry from one part to the other and just express themself differently.
It's too much to expect a Japanese manga (hell, a lot of forms of media) to have a character that sleeps around and not have them do it responsibly/without any sort of trauma that makes them that way. Maybe just a girl who enjoys sex and likes doing it but always respects boundaries or people's comfort levels or whatever. Now, obviously this is a manga about solving people's issues and whatnot so of course it would be tied to issues she has but I really really dislike how endlessly manga makes judgments about sexuality and what is and isn't "proper" or "right" often at the expense of someone's characterization so that it feels very unnatural. The MC is definitely the type to say something like "You should only be making the s-s-s-sex with someone you're in love with and want to spend your life together and have 20 babies" but the problem is that because of how this is framed and the type of character this new girl is...he's being presented as being in the right. If she was just someone who slept around but didn't have these negative ways of doing it than it may not make the judgment but as things are it really is saying a whole hell of a lot. I have little doubt he's going to "cure" her and she'll stop sleeping around once she's "better." Dude below me is right, her sleeping with people that are taken or targeting those in relationships instead of being responsible is the real problem here, not that she fucks.
Then again, this manga has often left things unsolved or shown that change doesn't really work like that so I might trust the author to handle it a little better. Guess we'll see.