My girlfriend has an ex that still want her back?

Seriously, this girls needs to know who she wants.
Tell her with all seriousness that you love her - but you don't want to be with someone who has got a foot in both sides.

You love her heaps, but do you trust her judgement? - she's had a horrible past and that she needs to learn to face it, accept the truth and grow up, mature and learn from it, and put it past behind her - otherwise she will jeopardize every relationship she ends up in.

If I was you, I'd be only friends with her for now. If she truly loves you enough, she'll think that she's losing you and will put an end to things with that other guy.
If you continue to love her just the same and not showing her how much you're hurt, she'll keep doing this - because she doesn't know how much damage she's doing to you.

Let her know all your pain and anger, but at the same time tell her that you love her.

Good luck.