Personally, I think it is admirable when we put our friend first and let them know something like Brandon knows, rather than not telling our friend because we are afraid that the person would be angry with us. I would have a hard time interacting with my friend, being in their life with their partner, and being genuine in that relationship and supporting her being with him, when I knew he had something happening with someone else/cheated.
If it were a close friend, there is not way I couldn't say anything. It would eat me up inside. I would be so hurt if my friend knew something like that and never told me. I would say, hey, I saw this...I don't know what this means, but I think you should know this. I would then leave it at that and not push her to end with him. Just give her the information and then it is up to her. I would support her after that, whatever she would decide. If she wanted to stay with the guy and want me to support that relationship, I would.
Just my opinion.