$6 From Bargain Basement Bicycle Parts (bbbp.net).
They also show a 4 speed Nexus with coaster brake wheelset for $50.
I've heard of tandem riders using a friction bar-end shifter for a third brake lever (hub brake for long descents), so there are ways to work two brakes with one hand.
To me a coaster brake and front brake seems like a better idea for starting out.
With modern wide range IGH or MegaRange cassettes I would be inclined to skip the front derailer for anyone just learning to ride, but if and when she needs it there are options: two thumb shifters; grip shifter and thumb or trigger shifter; grip, trigger, or thumb shifter and bar-end shifter.
I'm not sure what "25 percent functionality" means in practical terms.
This isn't something I have any experience with, but if you let us know a bit about her range of motion, strength, and dexterity I'll gladly give it some thought and offer any ideas that seem useful.
Last edited by NightShift; 04-24-12 at 07:54 AM.