Lost my best friend of 20 years to his cheating girlfriend of 6 months.

We've been best friends since we were 10. He was my best man at my wedding. We were brothers more than friends. A few weeks ago I caught his girlfriend of 6 months on a date with another man. She was openly kissing this man in the middle of a public space. I took a video and showed it to him.

He was furious and told me they were done. Then he goes to confront her and I don't hear anything until the next day and he tells me "it was a one time mistake" so he was going to forgive her, but that this was a betrayal and he didn't like me "spying on her like that." I pointed out the park they were at is right next to where I park to go to work. Her affair was during my commute for crying out loud! He believes that I'm in the wrong somehow.

It's been weeks and it's clear that our friendship is over. I'm gonna go drink and play some video games and hopefully get some sleep tonight. This has been eating me alive inside.