Just wanted to give an update on this. The wedding gig went really well. The bride had her friends choose the songs in advance, and the band had a separate rehearsal to brush up/learn a few. The songs they chose were:
Sunny (the bride singing this to her new husband, but changing the name)
Don't Stop (Journey)
Miss You (Stones)
Uptown Funk
F* You (Ceelo Green)
Jungle Love (Steve Miller)
Smooth (Santana)
Moondance
Hard to Handle
Lady Marmalade
I have say, it went really well. First, each person knew the song, and there really weren't any major screw ups. They'd done their homework. Second, some of the singers were really good...perhaps "ringers", as our band singer suggested. Third, the response from their friends in the audience was incredibly enthusiastic. It was for sure a highlight of the night. I could see why the bride wanted to do it. I would also say the bride, after the gig, gave us a great review, and on top of everything actually mailed the band a tip. And this is for a good-paying wedding gig to begin with.
I know this could have gone quite the other way had we not brushed up on a few of their choices, and had they not learned the song, but it was quite a success for the bride and her friends, so that works for me.
PS - the bride/groom were in their 20's, I think, and so were their friends (or possibly some in early 30's) so it's good to know some of our set list which has songs from decades ago were CHOSEN by their guests!