I had a lot of problems with hand numbness before I raised the bars on all my bikes and installed Nitto model 177 handlebars.

http://www.bikeman.com/HB1028.html

The 177(also known as the Noodle) is wide, has a flat ramp, and a slight sweep toward the rider. I have my bars set at saddle height, which keeps me from leaning forward and putting too much weight on my hands. The problem I had with flat bars is that even with bar ends there aren't enough position options. With the Noodle I have FIVE positions to cycle through(flats, ramp, hoods, drops, and hooks). Constantly moving my hands keeps the numbness at bay.

If the bike has a wide, cushy seat, that could be contributing to your numbness problem. I hate to be the inevitable Brooks saddle mentioner, but I've ridden thousands of miles on my Brooks B17 saddles and never even has so much as a saddle sore. They're worth every penny, IMHO.