I agree EX's should be off limits BUT i think it all depends on the friends and the circumstances.
I went to school with 2 girls (best friends) and they had their respective boyfriends, years later i meet them all again through FB, seems G1 left her partner and not long after so did G2, G1 is now married to and has a child with G2's Ex and G2 is nor married to G1's ex..
They never cheated, there were never feelings there before the splits it just so happened they were originally paired with the wrong person for them, they are now all happy, still friends and finally with the person they were meant to be with.

I guess sometimes we just have to realise 'you can't help who you fall in love with', no matter how much it hurts someone else it's your decision and just because someone wasn't right for me doesn't mean their not going to be perfect for my friend.

I personally wouldn't go there and i wouldn't appreciate it being done to me but in the end.. good luck to them all, if it's meant to be it will be!