Betraying my best friend - Do I tell him or not?

So to start this off, I'll be taking us back a few months ago.

My parents had decided to go to Italy for the weekend, leaving me home alone. For months I had been wanting to have somewhat of a party with my friends and this seemed like the perfect opportunity.
On the Friday that my parents left, I decided that it would be a good idea for me to sleep at my best friend's house as my parents were extremely stressed out (as they always are when they travel) and I prefer to stay away from them when they're like this. Whilst at his house, I had dinner with his family and we hung out for the night.
At one point during the night whilst in the bathroom he got a message on his phone and naturally, I unlocked his phone and replied. When he got back from the bathroom I asked him who it was and he explained that he'd been talking to this girl (let's call her Sophie) for a little less than a year and that he really liked her. He showed me a picture of her and whilst I didn't recognize her, I recognized several of her friends and made the decision that I would throw the party and try to get the two together. We had already made plans to go to the cinema the following day so I just invited a few guys and girls (including Sophie) to tag along and come round to my place for a bit of a get-together after.

I woke up early the day after to meet with our local weed dealer (who is a close friend of mine) to try and have him buy me some drinks for the party( I was 16 when this happened) and got him to buy me a lot of booze. After the movie we all came back to my place but before long most of the people had to leave. At this point there were only 8 people in the house and we were all absolutely off our heads.
I had been laying down on my sofa for a while and Sophie pops into the room and lies down next to me. This was the first time I had been alone with her and I finally managed to get a good look at her; she was incredible. I started talking to her and before I knew it, we were half naked and making out. This lasted for a good 20 minutes but it was at this exact moment that my friend walked into the room and saw us.
I felt absolutely terrible, but I felt even worse after speaking to Sophie's friends and finding out that she really liked him back and was just waiting for him to say something to her.
The day after the party I tried to talk to him but he was completely ignoring me, and he continued to do so for a few months after.

Now back to present day.

Two weeks ago I decided that after months of awkward encounters in the school hallways, I would try to talk to him. Doing so was one of the best decisions I've made as it turns that he missed me a lot too. I told him how sorry I was and he told me that he'd "forgive but not forget" and I respected that. I found out that in the few months that I'd been absent in his life he had formed a relationship with Sophie and they had started dating a few days after the party.
He invited me to go bowling with them and some of the girls from Sophie's school. I declined the invitation because I felt that it would've been super awkward between me and her. A few days later at a friend's birthday party I see her again and whilst I tried my best to avoid her, she eventually came up to me and we began having a conversation.

We talked for a good 30 minutes and I realize that I kinda like this girl.
Once I came to this realization I tried to distance myself from her for the sake of my friendship but that's when she dropped the bombshell
She told me that she couldn't stop thinking about that night and that she wanted more. Knowing that this probably meant meaningless sex I declined ( not wanting to lose my virginity to someone who was prepared to betray my best friend for something so meaningless) but she insisted that it wasn't meaningless and that she genuinely liked me. She said that after me, she couldn't think about anyone else.
Now this was extremely bittersweet for me to hear because on the one hand, this girl that I liked also had feelings for me but on the other hand, I had only rekindled my friendship with my best friend.

I decided that I would just stay as far away from her as I could for the sake of my friendship but I'm really unsure whether I should tell him or not.

Sorry for the long post, just really been wanting to get this off my chest.

TL;DR -
Hooked up with my best friend's girl; find out months later that she has feelings for me.
Do I tell him or nah?