Here's a possible option, if you really want to stay with her:
You tell her that you will get over this and be happy with her again as long as she; sits down right beside you, and sends a text to this guy in plain view for you to see, saying that she AND you are not comfortable with him texting her anymore, and that from hereon it stops. Stay with her for at least a couple hours so you can be there to catch his response to it and advise her again on a closure text if need be.
You probably think that's crazy, and yes I can understand why. But if she truly loves you(?) she will do it for you, and it will settle your mind lots. Perhaps just bring up this idea and see what her reaction to it is?