Does this "bad guy" also take photos of car show girls at low angle, flame people in photo forums, block people in event photography, always say people's photos not nice, in last minute cancel people's wedding assignment and pass comments that newbie's gear is not up to pro standard?
Unless the two of you are still good friends, chances are, she won't listen to you. Get a mutual friend or one of her friends to tell her, if she doesn't already know, but that seems highly unlikely if she is now attracted to 'bad boys'. Is/was she particularly over-protected growing up, or less experienced with the ways of the world, generally?
One other thing, speaking from personal experience, is, even though you don't want to see her hurt, which is very honourable, the fact is, you're no longer her bf aka 'protector'.
Aside, people have been known to swing in the entirely opposite directions when it comes to preferences after a break-up or long time relationship, or a change in life/lifestyle.
It's not so much what the person does that I would be concerned about, but possibly deeper-seated reasons for the swing. Is it just transitionary, or does it hint at darker reasons that were not apparent before? Aiyah ... my mind just working overtime lah ...
Seriously, you should be the ONE making the call. If you feel strongly that there is a dire need to "advise" her, go ahead. At the end of the day, it really does not matter whether your well intention is appreciated or not as long as your conscience is clear.
Have a victorious New Year ahead, and happy shooting.