What evidence does the swamper give that the ranch boss is a "pretty nice fella"?
All of the above is indeed true, however there is one tiny little detail that takes just a few words for Candy to utter. One day when the guys are in the bunkhouse, he recalls one Christmas when the guys received a special gift from the boss. It was a jug of whiskey. This must have been a tremendous gift to have received in a day and age when money was hard to come by. Candy is an older man and tells this little story reflectively and fondly as if it was one of his better memories in a long time.
As far as I can tell, the swamper (Candy) does not give George and Lennie any evidence to prove that the ranch boss (Curley's father) is a "nice fella." All he really says is that you have to "take him right." Presumably that means that if you understand him and the way he acts he is a nice guy. You can find this on page 24 of my copy of the book -- it is just after the boss has been talking to George and Lennie, trying to find out about them and what they are like.