August Kubizek is another individual with whom Hitler seemed to have sustained some bonds of friendship. I've read about him in Brigitte Hamann's Hitler's Vienna: A Portrait of the Tyrant as a Young Man, which perhaps is a very good overall source on the topic.
It was Adolf Hitler who, at the age of eighteen, successfully
persuaded Kubizek's father to let his son go to the metropolis to
attend the [Vienna Conservatory]. This, Kubizek wrote, changed the course of
his life for good ...
Kubizek saw Hitler for the last time on 23 July 1940, although as late
as 1944 Hitler sent Kubizek's mother a food basket for her 80th
birthday ...
When the tide began to turn against Hitler's favour,
Kubizek, who had avoided politics all his life, became a member of the
NSDAP in 1942 as a gesture of loyalty to his friend.