Most Helpful Girl
At this point and time, when an Ex appears out of "nowhere," this spells toil and trouble in a relationship that you are trying to be in with a "new guy friend." He hasn't gotten over what she did to him 5 years ago, so this makes him vulnerable in every which way. And more than likely when he sees her, it stirs up some old feelings that he may or may not still had inside. Now he is flat out---confused. Yes, you have every right to feel "insecure."
Continue to give him some space and time to try and get his head together. But you might want to contact him, from time to time, to let him know you are Still here for him. Feel the situation out. But don't allow yourself to be played as a fool. This ghost from the past may be here to Stir up a bit of a brew herself, and being she knows him better than you do, knows all the tricks of the trade on how to do it. xx0