ignore them. don't respond. don't dwell on it. keep telling yourself your friend is welcome to her and her cheating. It will only be a matter of time before she cheats on him.
Ashers, drink yourself stupid for one night only. Cry yourself silly if you need to. But then put it all in the history box and don't look back.
There are so many decent women out there, and one will come along when it's meant to happen. In the meantime just go out and make some new friends. And keep in mind that friendship often leads to romance. I know from experience my ex's have always been my best friend at the time. Someone to not only love, but to have a laugh with, to share some good and bad times, to kick me when i needed a kick (don't even go there BeHappy!

), and to support each other as we faced daily issues.
But most of all, learn to love yourself. Until you do that you can't really love anyone else properly. And you won't do it while you are focused on the pain being caused by 2 people who you really ought to consign to the sewer they appear to live in!