Should I tell my friend the truth?

Okay I have this guy friend who is like a brother to me named, Dave and his twin sister divine is like a sister to me. Okay Dave is talking to another one of my friends Kathy and they are getting really close, but today I saw Dave getting head from someone else, should I tell Kathy what I saw Dave doing? See my opinion is that since they are not going out it doesn’t matter but can you help me???

stay out of it... it's going to come back and bite you if you get involved...

I would tell kathy everything because she may think that they are getting close to dating and he may not be as interested as she thinks. just casually mention it before she gets in too deep to spare her feelings later.