Should I Tell My Friend that Her Husband is Cheating on Her?. protects our friendship and serves us both as best as possible and, I think,
Here's what you should do instead: Be honest. She's your best friend, so she'll value your honesty. Focus on support. Don't enable her. Don't waste your breath. Don't let it change your friendship. Don't change towards her partner. Set yourself some boundaries. Try not to judge.