<human>: *friend: Eric*

*girlfriend: Annie*

*female friend: Lana*

They've been dating for 18 months, Eric introduced me to Annie right away and we've become good friends. I've been hanging out with Lana for about 6 months and invited her to go camping with me, Eric and some other friends over a weekend.

During the trip Eric slept with Lana in our tent, I saw it happen, they didn't see me. I was angry so I took a walk and when I came back they were playing cards like they hadn't just been screwing.

I'm not sure what I should do, I wanna tell Annie because she's an awesome girl and I like her but I know that might blow up in my face. I've thought about talking to Eric and telling him to tell her or else I'd tell her but I expect he would be furious and stop being my friend out of pride or something. 

I'm not a guy who likes confrontation so this is quite scary for me.

Can you succinctly summarize the text in 1-3 sentences?
<bot>: My best friend cheated on his girlfriend with a friend of mine and now I don't know what I should do about it.