This is a tough one!
I see that you really love her and are heartbroken over her cheating on you with her ex.
I suggest you sit down and have another heart to heart talk with her and tell her if she cheats on you again that’s it!
The ex calling you to tell you is mean and sly and he seems to be acting out the Dog in the manger syndrome.
For that reason I’ll advise you to forgive your girlfriend but hold on and wait a little longer before popping the question to see if she cheats again.
When you are sure she has repented and changed then you can go ahead and marry her (provided you still love her!)