<human>: Could you distill the essence of the text into 1-3 sentences?

Disclosure: first post ever on Reddit. Apologies for formatting errors. 

I have been friends with Richard and Amanda for about 7 years and they have been dating for 6 years. I knew Richard first as he and I went to school together but I met Amanda shortly after when they started dating. They live together now, but are not engaged or married.  

Amanda and I have grown closer over the last couple years and she often confides in me her relationship issues with Richard. They are not in a good place but have been trying to work through things. She often questions why she is still with him. I try and give her objective advice and not take sides, but I know a secret I think she should know but I can't bring myself to tell her: Richard has slept with another woman. 

I found out though a mutual friend, Sarah, that Richard had slept with Sarah's best friend while we were on a vacation out of town (Amanda was not on this vacation).

I don't know what to do. I feel like I'm picking sides no matter how I try and fix this. On one hand, Richard and I are good friends and I don't want to ignore that and throw him under the bus without warning by telling Amanda what I know. On the other hand, I'm better friends with Amanda and I know if I were in her shoes, I would surely want someone to tell me. I feel like I need to be honest with her and tell her what I know.

I think the best course of action is to approach Richard and tell him that I know he has cheated on his girlfriend, and hope that he does the right thing and is honest with her. 

What should I do, reddit?
<bot>: My friend has cheated on his girlfriend who is also my friend. She doesn't know and I face a conflict of interest in deciding how best to tell her. What should I do?