3 words: WHAT A BITCH!!

It seems like they knew all along and didn't tell OP until the last minute. Otherwise, it would have casually come up in conversation that it was a wedding. If they knew, they should have said something so OP could've said he didn't feel comfortable with it, or at least had time to prepare himself mentally. Even if they aren't close friends, it would have been the decent thing to do.

That's so sad

Oh goodness, the irony in this. OP should include some sneaky phrases into their song hinting her cheating!

You should not take this job. Any human would understand your situation

You should of played kiss this! Or just not do it

Aw man, OP that really sucks. You're a much stronger person than I could ever be for even being able to go through with it without ruining the wedding by lashing out. Hope karma gives you a nice pat on the back and a much better future for the crap you had to wade through.

That chick is brutal! Hiring your ex to play your old song at your new wedding? Effed up man...vindictive douchenozzle...

You need a hug

wow...that's rough dude...