With all his affection towards Muggles for several decades, one may wonder if he even had any Muggle friends at all. He surely was acquainted with the parents of his Muggle-born students, but I'm curious if he had any actual friendship with a Muggle (where the Muggle knows that he is a wizard, and they spend time together fooling around etc).
I can't find any canon on this, but he grew up in Godric's Hollow and then went to Hogwarts. After that, he worked at Hogwarts for the rest of his life. He's never lived in a Muggle community, and as a kid, he thought wizards to be superior to Muggles. He has probably, as you said, met the parents of Muggle-born students at Hogwarts, but that is all he would have done, and they don't really count as his friends. He had no chances to meet any Muggles during his lifetime.