If you type Hitler as artist into Google, the first site that comes up has four supposed Hitler paintings shown for sale (three of them sold as of this morning). It contains a quote from your friend's book, so evidently it is known in the trade.
http://www.pzg.biz/hitler_the_artist.htm
There was a controversial exhibit at Williams College two years ago, described as follows:
'Prelude to a Nightmare: Art, Politics, and Hitler's Early Years in Vienna 1906-1913,' an exhibit of 150 paintings, drawings, watercolors, prints, posters, theatrical designs, vintage film footage, photographs, books, and pamphlets.
One assumes that at least some of those 150 objects were actually Hitler's work.
all the best