Neighbour vs best friend

When I threw a party, I invited all of my friends and my 2 neighbors because they are our age.

My best friend likes this guy. She is CRAZY about him! She talks about him all the time.

But at the party, he was flirting with my neighbor the whole time, like talking, laughing, and dancing. Then he talked to me about her the next day and told me he's going to ask her out! My best friend will be SO PISSED!

Should I tell her, and ruin our friendship? Or let her find out herself?

well how will that ruin your frienship, it wouldnt. just tell her.