My son's girlfriend is been cheating on him with his friend ??

Both she and my son are 25 and have been together for just over 9 years.I received an e-mail from someone who has sent me evidence that she has been cheating on him for the past 5 years with his BFF. I talked with her about it and She convinced me and his sister to not say anything . Most of the time it happens it when hes out of town town or at work. I know that she cheated on him last night. I have NO idea how to handle this. Hey Parents out there as a Mom ,should I say anything to my son or just let it work itself out and he will figure it out soon enough?

Well its risky to say anything as you might end 9 year relationship. But on the other hand things are not right now either. I think his GF should tell him. So tell her either she tells him or you will do it.