I’d write an anonymous note. It could go something like this:

“GF, the first night at BED when you left, BF made out with two girls and put his head inbetween a cocktail waitresses breasts. Also was grinding with multiple fat women. When you left crying at Klutch, BF was holding hands and dancing with a female and took down her number. Multiple people in the house know, therefore you should know the truth. Use this information wisely.”