if you want to find out about what they are talking.. I suggest putting up your MP3 (if you have one) on record mode (make sure it can record long, and somewhere she can;t find it, but close to the phone..). you can trace everything back. in your case I would be supsicious as hell >_<
check her mail, (you should know her password) and see for any messages for him. check instant message history from any application and do w/e it is to find out what's going on.
I would aswell check up on her sometimes if they meet up together. keep a close eye on them.
it might look getting a bit far but.. it's for your own sake. you don't want to see her walk of with that friend. you'll hate him FOREVER.
hope this helped you a bit ^^