If this is happening to your buddy, then you need to set his ass straight.
Don't get me wrong. I'm not exactly a role model in all this. Had a girlfriend and a booty call I worked with for a while. Booty call knew about girlfriend, girlfriend didn't know about booty call. And if girlfriend ever found out about booty call I would have expected her to dump me. I had to end both when girlfriend said she loved me and booty call wanted to date me. That was some serious withdrawals!!!
It was fun for a while though. Especially when I'd have to tell booty call that we couldn't fuck today cause I have a date with my girlfriend.