I know what I did was bad. I feel bad because this is one of my good mates from home. I was out at the pub with some friends and a group of girls are next to us, when I see this blonde girl. She comes over and starts talking to me. We chatted for a good while and I asked her if she had a boyfriend, she said no.
So then we took our group to one of her friend's flats, and started to drink a bit. I'm not much of a hardcore drinker so I was drinking very minimal in comparison to my other friends. Then and this girl start to get cozy and she tells me she wants to just relax with me alone. I was down and then one thing led to another and we slept together. This was 1 week and a half ago. We've been chatting since. she told me she was heading out of town to visit some friends, so we couldn't hangout. I was on snapchat and I went through my mate's story when I see her in it and he's leaning his head on her. I was shocked. I messaged her and it would be too long to explain here.
I told my mate that I didn't know she was his girl because I had not seen her on his social media and plus I only see my mate whenever I'm on break. He's pissed of course but he's angrier at her for doing what she did. I'm trying to make amends with my mate, how can I?