Something like this happened to me. I called the girl, emailed her, and texted her telling her to back off... never to receive a reply. She hasn't contacted him since though. I confronted him about it and he said he didn't think I would have a problem with him talking to her and hanging out with her when I was at work and he wasn't and "Baby, do you really think I would cheat on you?". After I said what I had to say, he understood where I was coming from and how I felt like he was sneaking around because he never even mentioned her name to me. I mean, if it had been me texting and hanging out with another guy without telling him, I know he would have been just as upset, if not even more upset than I was.
I decided to tell her off first because I had a feeling if I asked him to stop texting her and hanging out with her, he wouldn't actually tell her why he couldn't hang out or talk, so I figeured what better way to end it? Let her know how pissed I was when I found out, so she understands my side of it.