I honestly can't remember what I used to call the parent's of my friends. Maybe it depended on how close the friendships were. I know I called my best friend's mum 'Mum 2'. My friend and I were so close and I was always made to feel as part of the family. Her brother called me 'Sis'. Other friends I think I must have called them Mrs SoandSo or by their first names.
I grew up in the southern part of the US and know that many of my friend's children now call me Miss Mum or if I was male it would be Mr Mum, and while that might be strange to a lot of English people for 'Deep South' Southerners it is the done thing. We also address our elders with the terms 'mam' and 'sir' as a mark of respect. I find it disrespectful over here when addresses as Love, Darling, Duck, Mate, etc.
My DS's friend generally call me by my first name, though sometimes it can be a bit confusing as I work in the school that some of them go to. So at school I'm Miss Sons and out of school Mum, occasionally there is the slip of the tongue in the classroom.