My girlfriend won't stop texting this guy

I've been dating my gf for almost 10 months now, we love each other and are now engaged. But she texts this guy off and on, not everyday but occasionally. He used to like her and she liked him to at one point. He flirts with her and she doesn't seem to see why I get mad, am I wrong? Or should I not care. And I can't stand him. Please help.