Am I being overly cautious?

He's probably still into her. If he was that obsessed, I doubt he'd give up that easy. He's probably just backing off for a while, or trying to pretend he doesn't like her to save their friendship. I'd be very weary, personally. Ask her what she wants to do about it. However, be careful that you don't come across as possessive. Fine or not, if you push your worries about the guy too much, or if you directly confront him without talking to her about it first, you could just sound jealous and possessive and cause a fight between you and her. I'd walk with caution around the whole situation. It sucks, but maybe you should just keep an eye on him for now.