How can I be happy for him with this girl?

I had two close friends. One of them is a married girl and the other is a single guy. When her hubby is away for business, we used to hangout alot. At the same time I was hanging out with this homeboy all the time. She left the town with her hubby but came back until hubby gets a job but after many arguments, she decided not to go back to his hubby. Somehow my homeboy and she got hooked up right after that. What I didn't understand was it was obvious they were more than friends but they waited for the right time to tell me about the situation, they said. now I feel betrayed and can't face them like I used to. I thought they were my best friends and would tell me anything. now my homeboy hardly talk to me.