I find that a nice line of tequila shots help women fall out of love really quick, and then fall IN love with eachother.
????
PROFIT!!!!
But seriously...don't you get involved with that shit. Don't date either of them, because if your friend decides he wants her back, she'll leave you for him. EVERY ****ING TIME! (I've been screwed over like that at least 3 times. Never be the recoil boyfriend.)
Also; it just takes time. They'll get over it real quick. Just point out what an asshole he must have been to do something like that. It always makes me sick when I hear "Oh hey, we're done now cause (so and so) is better".
It just shows the whole relationship was based on something that wasn't solid to begin with.