Dude... Nothing is going on. Relax. If it was, you'd have more to be suspicious about than a few texts.
My ex and I were best friends. We spoke every single day, often more than once. Her boyfriend became a close friend of mine, my girlfriend didn't even question it. Half the reason we broke up is because we realised we were better as friends than as a couple. Maybe your girl and her ex found the same thing.
Don't be jealous, don't be a dick about it, just try and be chill with him. In fact, why not suggest inviting him out for a beer so you can get to know him as a mate, too? If you're mates, he'll be less inclined to make a move, so it's win-win.
I'll leave you with passing advice paraphrased from Jay-Z:
"If you're having girl problems I feel bad you you son, but you've got 99 problems: Being a ***** ain't one."