You could have some fun with this one -- Just walk in front of them, casually. Pretend NOT to see them, but make damn sure they saw you! Then have loads and loads of fun with the conversation the following day, or two, or three.
It's possible that this was just some random girl, by the way. Which you could also find out, using the above method. (He might just be like, "oh i was at the mall with my friend/cousin [name] and...")
WOW. I can't even imagine how angry I would be. WOW. Break up with him over text. He doesn't deserve a respectful breakup anyway so just type up something and hit send. Or bring up the situation over text that's as easy as it'll get.