Even if YouTube's algorithms don't take it down, you are risking a DMCA takedown notice at best and a lawsuit from the copyright holder at worst.
There's less risk of this happening on
Vimeo
- but,for their own protection, they're serious about this stuff too:
http://vimeo.com/help/faq/legal-stuff/fair-use
People post copyrighted music all the time, but, in my view, it's not worth the reputational or financial risk to your business (or to you personally, if you're not incorporated).
Maybe you can give the couple the edit with the music track they requested and post a separate YouTube edit with a track from freestockmusic.com or a similar service? I use it, it's absolutely free, and the tracks are decent.
I'm no lawyer, so this advice is worth pretty much what you paid for it :)
Bill
Hybrid Camera Revolution