well, in the "man that guy leads a pathetic life" category, I'd actually met a girl who sang and played uke before this summer and spent all summer finding stuff for us to do, including duets, before I met up with her again in the fall. We met up but only on accident because she was playing with the only other ukulele player in town and she's blown me off about 6 times since then. There's a lot more pity inspiring event following but I'm off topic. In any case, here's what I found for male/female duets
Anyone else but you (from the Juno sound track)
it's super easy, sweet and enjoyable
song
http://www.youtube.com/watch?v=-N3BjVMWziE
tab
http://www.ilovefizzypop.co.uk/wp-content/uploads/2008/05/anyone-else-but-you.pdf
The one everybody's brought up (Tonight you belong to me)
tab
http://ukulelehunt.com/wp-content/uploads/2008/06/tonightyoubelongtome.pdf
instructions for tab
http://ukulelehunt.com/2008/06/15/tonight-you-belong-to-me-the-jerk-version/
In 5 Years Time by Noah and the Whale
song
http://www.youtube.com/watch?v=T8YCSJpF4g4
tab
http://ukulelehunt.com/wp-content/uploads/2007/12/fiveyears.pdf
I've got more upon request, but other than that I think the most successful you'll be is from one of two other strategies:
A lot of people don't want to go through the hassle of learning the lyrics for a whole song (unless you're inclined on performing with him)so just pick a song you think he'll know and learn that one, then figure out a duet formation of that. Or, if he's a performer, learn a song he already knows, I learned a song that the uker I'm talking of wrote (with the help of another UU'er), she seemed happy about it.
In any case, I wish you well with your duets.