Should I tell Brandon’s girlfriend about her boyfriend's promiscuous behaviour?

I got to hang out with Brandon and a few of his friends last night. We were at his friend’s place, watching the game and ping joints around to smoke. Brandon, who has a girlfriend, called this other girl on his cell phone. A few minutes later, the girl came over and she and Brandon went to the room next door to have . After Brandon finished and showered up, he re-joined us and said, “I’m going to call my girlfriend to see what she’s up to.” Brandon is such a lil playa and his girlfriend is completely oblivious to this fact. When I first hung out with Brandon the other week, we went to a club and he hooked up with a really hot girl. They left the club together and I can only imagine what happened afterward.