Because Kelly left them as she didn't want to be with them anymore, is that why she deserves what Danny did to her unless you are kind of saying she cheated on all of them??
And if any of them are really happy that she has been cheated on by Danny after having not been with her for quite a while (maybe not so Thom) then they really sad men.
Well she did cheat on Jason Statham with Billy Zane.
I don't think the poster means that they are happy she has been cheated on, just that they are happy in general.