Because he already knows what the girl is doing to him, he may be holding out hope that she will ultimately fall in love with him and not go back to the other guy (which may or may not happen). He probably also really like the nookie he’s getting from her whether she loves him or not (guys tend to be able to separate love from getting laid, more than girls can).
Like some of the others have said, you could sit him down and say, “Burt, I know you have feelings for Brittany, but she already told you that she’s in love with Braxton and he’s coming back and you know she’s going to get back together with him. I don’t want you to get hurt.” Then let him talk.
Burt probably has fantasies that Brittany is going to fall in love with him. Or he doesn’t really care if she loves him as long as he gets some good nookie from her until this other dude shows up.
Brittany probably likes Burt just fine, and she’s probably the kind of girl that cannot be alone or without a guy for one second. She’s probably aussuaging her guilt by telling Burt up front that she loves Braxton. Even though she knows full well that she will dump Burt the minute Braxton shows up. But she’ll feel sorry for Burt and hope that they can still be friends (isn’t that f*cked sweet?)
Braxton probably doesn’t know that Brittany is keeping company with Burt and he probably won’t ever find out, Brittany won’t tell him and she know that Burt won’t tell him (because then he’d lose his nookie privleges) and Brittany will drop Burt like a hot rock the millisecond Braxton comes back into town. If Braxton did know about it, he’d probably dump Brittany and immediately hook up with Courtney, Brittany’s other BFF (isn’t that f*cked sweet?) Courtney will try to explain to Brittany that she shouldn’t have cheated on him. Brittany will scream that BFF’s should not date each others exes!
The other thing you could do is talk to Brittany, herself, and ask her why she’s hooked up with Burt even though she’s going to go back to Braxton and see what she has to say. She’ll give you some song and dance about how she likes Burt, but she _she doesn’t “like-like” him. (isn’t that f*cked sweet?) Also, if and when she does dump Burt, which is pretty much inevitable, if Burt finds out that you discussed his situation with her, HE WILL BLAME YOU FOR THE BREAKUP and you will likely lose him as a friend.
So your best bet is to be there for Burt when Brittany dumps him, but be prepared for weird situations, since you will still be friends with Brittany, right? (this part is really sweet f*cked!)