You either do or you do not. Not everyone has it in them to trust others. I, for instance, am absolutely incapable of trusting someone in a relationship, which is why I am perfectly happy being single.
I read recently on one of those stupid internet quote sites "Don't judge me by my past. I don't live there anymore." You give EVERYONE a clean slate. Holding something against someone that isn't their fault is very unfair. Maybe you should remain single for the rest of your life.
Anyone deserves a chance to prove that they have admirable qualities. Prejudging precludes reality,in most cases. Give her a chance and judge proven results,not anticipatory fears.
If you are going to judge everyone you are with based on someone you were with in the past you are never going to find anyone to have a strong secure relationship with.
If you stop thinking of women as bitches, there is a nice start. And, if every woman you date becomes a bitch...aren't you the common element?
My advice is that the guy should use the logical part of his brain and create "New file" for the new girl and judge on a case by case basis. You ex or previous girl doesn't exist anymore for you. Use "delete file for memories and emotions attached to those memories.
Allowing past hurts to influence your current situation could stop you from falling for a really amazing human being. Sometimes you just have to jump in and hope for the best.