I'm writing the following under the assumption that you're wife's bike has a coaster-brake rear hub and an OPC (fairly standard cruiser set-up)....Normally, I'd recommend going bigger out back, but unless you really hunt for it (
http://www.bikesmithdesign.com/cogs/index.html), you won't find a snap-ring cog bigger than 23t. I think that a 44t/23t ratio is plenty low enough (49.7" gear), but obviously some folks like to spin more than I do. It is easier to drop gear-inches on your set up with a smaller front sprocket. If you go too small, it'll look crazy on a cruiser (just my opinion, but it's one shared by many.) I'd suggest going 40t front with a 22t rear. This'd give you 47.3 gear-inches, and can be had for cheap.
http://www.amazon.com/Wald-Steel-OPC...words=wald+40t $6 sprocket
http://www.amazon.com/Sturmey-Archer...+Cog%2C+Silver $6 cog
You might want to swap the cogs first, and run the 44x22t ratio and see how she likes it. (Hopefully, your chain is long enough and there's enough room in your dropouts to accommodate the difference.) If that's not good enough, go ahead and switch the sprocket, too. Switching either only takes minutes, with a flathead screwdriver and a big adjustable wrench being all the tools you need. Of course, once you got the OPC apart, you'll prolly wanna repack the BB...
hth