Take some of the work to one of your local galleries (not a mall frame shop or an A.C. Moore) and tell them all you can about your friend's uncle. Do not leave the work there. They may be able to suggest an outlet or a dealer, or a better suited gallery, depending on: where you live, their particular definition of outsider (it changes everywhere you inquirer, it seems) and where to go from there.
Document what you can, either scans or photos, with dates and titles and mediums, as best you can.
Although some "outsider" work has been "in" for the last decade or so, it's the story that goes along with the creation of the art (see Henry Darger, Howard Finster,) which drives demand, and therefor, prices. There is no market one can count on like one can, say for example, Rock Poster art of the 60's or Early American Glassware. Those have defined and well established parameters. The buying and selling of so-called outsider art is a fickle and sometimes fairly indifferent business. I've seen well established and highly educated living artists having damn-near retrospectives at the American Visionary Art Museum (a museum set up to showcase said outsiders, visionaries, the self-taught, etc.) in Baltimore along side total unknowns who lived and died in obscurity and left behind 4000 paintings for the landlord to sort out. So, you see, it can get complicated.
Good Luck! And Good Karma to you for trying to get your friend's uncle's work seen..