I only know one father who, when we were children, swore the air blue at his offspring.
He is the nicest man. He never raised a hand to them (unlike my father who terrorised me regularly), he made sure he was home from work by 5pm every day (no mean feat, as a builder's labourer), and both his children are now happy successful adults, and he is now the most loving Grandad Mick (as he was Daddy Mick to all us waifs and strays who took refuge in their tiny, grubby, smoke filled animal smelling loud sweary house).
I adored him then, I adore him now. He was one of those parents who can be both a haven and a friend - we could always go to Daddy Mick. And he would always let us in - he hung a key up for us in the toilet (which was one of the last outdoor toilets in this country)
The times I ran to that house from my own Victorian four bed semi, sobbing about how my dad kept hitting me and my mother didn't love me enough to stop him. The times he poached eggs for me because I had stayed over night because my house was a seething volcano. He used to bellow up the stairs to us "If you two don't get your fuckin arses down these stairs these eggs are goin' in the bin, I'm not yer cuntin' slave!!!"
he never threw our breakfast away. Well, he did once, it had gone cold, but he made us some more. Conversely, it wouldn't have occurred to my parents to make me breakfast once I could reach the cereal.
I guess what I'm trying to say, through all this drabble, is that the most unlikely people can be pure gold. And Daddy Mick was pure gold, pure pure gold. Maybe your daughter's friend's family isn't like this - but maybe they are, and if you stop your daughter ever seeing them, she'll never have the chance to experience a different, but still good, way of life.