How to get two people to hate each other (just read it!)?

I have two bestfriends. They just met, and my girl bff is starting to fall for my guy bff, which is gonna suck. He's a manwhore, and he is the biggest player. Not even joking. I've already told my girl bff, but she won't listin. Ive told her a million times. I know they are gonna get together soon, and then he's gonna dump her on her ***. I need to get them to hate each other, without the each other knowing im doing it. I'm just doing it to save my friendships.