I can understand a mum being concerned about their child and whether they are making friends in secondary, but Sarah's mum has gone about this all wrong.
Some parents are so focused on their own child, combined with poor self awareness, a touch of entitlement, and you have a parent like Sarah's mum thinking she can dictate what another child does during their school holidays.
That she would text OP, a parent of a child she was never historically friendly with is hugely presumptuous.
I remember getting a text from a mum telling me that she was only allowing her son to attend a concert with their large group of friends because my son was going and would look after him.
She meant regarding drinking, which her son was fond of ( my son liked a couple of beers too)
I text her back that she should rethink that plan because my sons priority was himself and not babysitting anyone else, and I would be making that very clear to him.
I told my son under ANY circumstances he was not to be buying any alcohol for her son who was just under 18, as he would have me to deal with.
He was being appointed minder 2 hours away from home at a concert in another city.
I very rarely put my foot down so he knew not to mess with me.
Of course they look after each when out, but absolutely no way was any parent going to be formalising that responsibility onto my child just because she decided it suits her.
My son went straight back to him and told him what his mother had done and what I had made crystal clear to him. He told him he wouldn't be buying him any drink while they were away.
Absolutely take care of each other but I am very firm with my children that there is a difference between looking out for each other, to becoming friends minder while they get drunk every night, secure in the knowledge their friends will mind them.
One of my daughters pulled a bit back from two friends as this happened once too often and my daughters night out was being completely spoiled. Fortunately after a few nights of her telling them she would see them there later got the message across that she was no longer up for it.
Being pulled out of class is not done casually in our schools so it would raise a question from friends.
Fortunately the teachers in our schools are all over bullying, but know better than to be insensitive by calling out very young children out to be spoken to.
The average 11 year old could feel quite intimidated by being called out like that, far more than a much older teen would.