If they were happy with the other person, have just a simple date with them then leave them to their destiny. Now if they were marrying the other and he wasn't exactly well suited but the only option left (Fiona in the first Shrek movie), I'd do all that I could to steer her back to me. State what went wrong, why it did, and how we could fix and learn from it.
WWYD if someone took something you said the wrong way, and they won't even accept your apology for it, yet they're good friends with the others in your group?