Bad friends

A matter of days ago, I found out that a friend of mine, 'L', has been cheating on his girlfriend of 2 years, also a friend of mine, 'K', with our other friend (my ex-flatmate) 'B'. I found out when another girl in our friendship group slipped up thinking I already knew. When I asked for more details she said that it had happened frequently a few months back, but she wasn't sure whether it had stopped or not. She also told me that a most people in our friendship group knew, which personally I find makes the situation a lot lot worse. Even my boyfriend knew (he's best friends with L. Although he knows both K and B, they're not as close). We're a very close group of friends, but the fact that no one has said or done anything about it for so long really makes me doubt their sense of loyalty. I'm 20 and have known K for 8 years, since high school where she was the year above me, we both moved from Scotland to an area outside London for college 2/3 years ago where we met our previously mentioned group of friends. Although I wouldn't say we are best friends, and have a very close relationship, we definitely have a sisterly and very protective instinct over eachother. We both became good friends with B as she was also scottish and became a bit of an add-on to our sisterhood. At one point before B and I started living together, I'd have said K and B were closer than I was to either of them. I always admired K and L's relationship and wouldn't have ever thought either of them would do such a thing as cheat on eachother. They seem very much in love, although neither of them are the kind of people to openly talk about emotions with friends. I suppose it's similar to when a kid idolises their parents relationship, and then when they find out they're getting divorced it sort of throws all that in the air and makes you doubt it all. My parents split up before i was born so have never experienced that but it's what I relate to with regards to how i feel about this situation. As for B, I always knew she was a bit sneaky with men. I wouldn't be surprised if she told me she had slept with someones boyfriend, but I'd never in a million years have thought she'd do it to a friend, especially not K. My dilemma now is what I should do. This knowledge has been troubling me since I found out and I'm totally disgusted no one has told K already. My boyfriend says 'It's not our place to tell her', but I know that if he was cheating on me, I'd want my friends to tell me, and I'd be extremely upset if I found out they knew and didn't tell me. I just know this will destroy her and I don't think I can be the one to tell her. I keep thinking about the saying: There are people who do bad things, and then there are people who see bad things and do nothing about it. I don't want to be either of those kinds of people, but I don't want to end up having this backfiring on me (eg. she doesn't believe me and no one will back me up). Spoke to my mum and she said to confront both B and L and tell them that if they didn't tell K, I would. Wish I had my mothers backbone. AAAAAAARGH.