My DC (9) was talking to his MIL on the phone about his afternoon at his friend's house and said 'Then Brian took us to McDonalds!'. MIL asked who Brian was, when my DC told MIL it is his friend's dad she said it was very odd to call him by his first name, not Mr (last name).
My DCs have have always called their friend's parents by their first name or '(friends name) mum' if they don't know their name yet.
Using Mr & Mrs (last name) seems very american to me and I could only imagin my children using that if their friend's parent was also a teacher at their school.
Is this a generational difference or maybe location dependent?
What do your DCs call their friend's parents? YABU - They use Mr and Mrs (last name)
YANBU - They use their first name