(Fake names used)

So, to put it into context, I (20F) met my best friend Joanne (19F) when I was 14 and she was 13. We met via Discord, and since then we've been inseparable, despite being quite far from each other (I'm in Newcastle and she's in Birmingham), but we're really close nonetheless. I don't know what I'd do without Joanne, we've pretty much watched each other grow up, we've seen each other through heartbreak, family struggles, deaths, etc. She is truly my person and I wouldn't trade her for the world.

Last year, Joanne met Ernest (19M) via Discord and they started dating. Ernest only lived a couple of hours away, but he would refuse to drive up to see Joanne, despite her attempts for them to meet in person (Joanne also couldn't go up to visit him as she didn't have a car). They weren't a great match, they fought all the time, and eventually broke up.

Me, Joanne, and Ernest had a groupchat together, and we were talking about adding a few people. I added my girlfriend, Joanne added her friend, and Ernest added his best friend Andrew (20M). Joanne and Andrew hit it off, and after confirming with Ernest that it was okay, they started dating. The distance between Joanne and Ernest and Joanne and Andrew was the same, but Andrew would drive down to see Joanne every couple of weeks and seemed really dedicated to her.

After maybe two months, me, Joanne, Ernest, and Andrew all agreed to meet up for the first time in person as a group down in Birmingham. Unfortunately my cat got sick the day before I was supposed to go down, so I had to cancel. This led to just Joanne, Ernest, and Andrew meeting.

From what I've been told and from the pictures I've been shown, they went bowling, went to Dominoes, and went to HMV. I was still quite down that I couldn't have been there, so Joanne called me once she got home and told me about what happened. When she was telling me about the bowling, she got really quiet and looked guilty. I asked her what was wrong, and she admitted that when Andrew was bowling, Ernest had kissed her. She said she didn't want to pull away to avoid making it awkward, so she kissed him back. She was crying by this point, begging me not to tell Andrew, saying it was a mistake, it'd tear the friend group apart, and she'd never speak to me again. I told her I wouldn't tell Andrew, but now I'm torn.

This was 3 months ago. We've met up as a 4 since then, and Andrew's lovely. He's a really good person, a good boyfriend to Joanne and an amazing friend to Ernest and me. I wasn't close with him before, but we have a lot in common and we'll sometimes have deep discussions about life and such. I have absolutely no romantic feelings for him- he's more like a brotherly figure who I really care about.

Him and Joanne are still together and he has no clue that her and Ernest kissed. I'm the only one who knows about it, and I don't know if I should tell him or not. I care about Andrew a lot, and I don't want him to be living a lie, but as Joanne claims, it was only once and Ernest came onto her. If I told Andrew, it wouldn't only destroy him and Joanne, it would destroy Andrew's friendship with Ernest, Joanne's friendship with me, and destroy our entire friend group. Please help because I'm not sure what to do here.