Questions about friends

This is going to be a little complex and rambly, and I'm aware that this won't be anonymous if the guy finds out but that's half the point (cough). I am in the Sixth Form (age 17) at an all-boys' grammar school (yes very homoerotic I know). Since Year 8, when I was first truly aware of my sexuality, I've shared a class with someone who I'll call Lewis here because I don't know how strict the mods are on real names and also because he's called Lewis. Lewis was and is very attractive and was also my first crush. He and I used to row together and we are among best friends now- the only 'problem' is that he has a girlfriend, who also happens to be one of my friends. So when he approached me a few days before our 4-day Geography field trip and asked me if I wanted to sleep with him and share a room together at the centre, I said yes initially. Bear in mind at this point I've been fantasising him on-and-off for the best part of 5 years here. He says that he has always been curious about anal and wants to know how it feels, and that because I'm a good friend, he trusts me (and also I have a fair bit of experience somehow; I don't know how, I'm not exactly attractive). He is serious about it and to be honest I think I am too; and he is going off to university at the end of the academic year so naturally we will grow apart to some level, and of course I've wanted to sleep with him for some time now, but the two of them make a wonderful couple so I am naturally rather nervous about the outcome or if she finds out (she doesn't Reddit so I'm safe here). Have any of you had similar experiences? If so, what do you recommend? Should I do it or have moral standards? Thanks for feedback :)