Tell her to stop being such a dumb whore and unless she wants to end up as one of those pathetic women who gets beaten then tells everyone she falls over then she should jolly well stay the **** down there.
Also show her pictures of deadbeats because thats what she will end up as if she is some some turd horse.
Also send her old bf an email telling him he is a wasteman and that instead o hitting women he should come hit you.
Sometime shock talk like this would probably work, but only in about 0.0001% of the time, for the rest of the time you would probably get a 2 word answer in reply with the 2nd word either being you or off.
I have seen this happen to one of my wome friends when I was at uni many moons ago, and that experience taught me sometimes it is just best to not get involved up untill the point where you get flat out asked to help / for advice.
If you do end up having the talk anyway, the best way to approach is to do as little of the talking as possible. Let her get eveyrthing off her chest and then when she asks you directly what you think, keep it short and sweet.
Remember it is human nature for people to not lik ehearing what they don't want to hear more so if it is the truth.
Bottom line getting involved in someones personal life always ends up fubared for you regardless of if it was done for the right / wrong reasons.