Replying to I slept with my best friend's wife.

I need some help here I have no idea what to do. My friend is in the navy and his wife calls me when he is out on service and she needs help doing jobs around thier house. She called me Saturday around 5 to see if I could help her out so I went over and after the job was done she was like want to have some drinks, I said sure, well we drank a little to much and I can recalls bits and peices from the night. But when I woke up I was in their bed with her and we were both naked. She was already awake and she looked at me and said that was fun. I said we didn't did we , and she says back we sure did. I start to freak out and she said oh take it easy your not the first guy that had spent the night here. Meaning she sleeps with other guys when my friend is away. So I'm stuck I can't ask my friends for help because we all know each other. How do I tell my friend his wife is cheating I him and I was one of the guys she did it with ?