To feel upset me and DS have lost friends and it wasn't our fault

It's happens all the time.

10 is usually the age kids develop very differently in regards to puberty and emotional development, hormones and trying out new things.

Sometimes that means they become complete arseholes in the process until they realise it doesn't make you friends. And at 10yo that sort of behaviour won't get as much short shrift as it would older or even adulthood because the other kids don't have the emotional intelligence, confidence and language to call it out.

But I have to say - it's really nice to hear of a school take an absolute no nonsense approach to bullying. And obviously it's worked because she's shut up.

Just tell your ds that life is for having a hide variety of friends from all walks of life. As he grows up he'll meet people who become short term friends and some long term.
Forget this relationship and spend time with your other family friends.

Maybe get ds into sos e kind of club to meet other people outside of school.

But he had to learn as well you can't control another persons behaviour - just your reaction to it.

I think you need to be honest with your poor son. You can say I don’t know what motivated her to do this but sometimes people do and let you down. Also explain that when people are growing up this is even more common. It looks to me like Mia wanted to join the cool kids but didn’t know how to tell your son.

are there other children you can help your son develop a friendship with?

it is ok to show your son that you are also hurt, he will look to you so model being hurt but making other friendships and moving on.

ah, sorry this is happening to you both x I would say at this age, friendship groups can really change. Hormones, growing up at different rates etc. My own DS had a best pal since nursery, and come P7 (Scotland, he was around 10) suddenly the besties were no more. his friend was saying harsh things, and excluding him. He was very upset and found it hard to navigate. I did tell his class teacher, but she was great, and said she'd keep an eye, as DS was very worried about being seen as having 'told'.
They have now worked it through themselves, and are OK, but not besties. They are up at high school now anyway, so that will shake things up for most friendship groups.

I'd suggest encouraging other friendships if you can. And obv, the mum will 'side' with her own child, and might be embarrassed? I found that my DS' besties' mum hadn't been aware of how bad it had been, and I guess, from her perspective, it hadn't been. Her child hadn't been at home in tears after all.

hope things improve soon x

Well, surely this suggests that in fact you haven’t lost two good friends, your son was just the victim of the kind of messy and upsetting friendship switch that sometimes happens with young children, and you realised that your friendship with the other child’s mother was purely situational, and didn’t survive the fallout?

An actual friend would have been concerned and promised to talk privately to their child to figure out why she’d turned against your DS. I had a not dissimilar situation earlier this year with my DS, aged 11, and talked to four parents in his friendship group, all of whom I also considered friends. They talked to their children, and the behaviour stopped.

I know that this isn't what you want to hear but you and your ds are better off finding out what sort of people Mia and her mum are now and getting out before that little madam gets worse and drags him into even more drama because in her mum's eyes Mia will never be wrong it will always be someone else who's to blame, and we all know what sort of brat that creates

You poor things! Unfortunately, children can be pretty brutal about ending friendships, especially when social status comes into it and they start to become more aware of that. If Mia has been drawn into wanting to be in the cool gang, she may have felt that, to be accepted, she has to be seen to reject and dislike anyone who's not in the cool gang. This kind of thing is not uncommon. As for Mia's mum... well, parents will often react very angrily and cut off contact with anyone who criticises their child, rather than believe their child has done anything wrong. I hope your ds finds another, nicer friend!

This sounds really tough for both of you. Sometimes, I find, you just need to tell the truth and treat your child as an adult.
Tell him that sometimes people do things that are unexpected and friends let you down. We should do our best to understand them and forgive them but we can never really know why people do and say things that are hurtful. The best thing we can do is to have a conversation with them and find out if we upset them in any way but, if it doesn't work, we just have to accept that the friendship is over, forgive and move on. In time, they may realise their mistake and apologise. We can then decide whether to accept the apology and continue the friendship or whether to accept the apology but distance ourselves from the person anyway.

It is very normal at this age for friendships to change.

It's unusual for children's friendships to last for many many years - it's more normal that they have fallings out and fallings in.
Most parents of 7-11 year olds will have experienced their child coming home and saying "I hate X she's so horrible" and the next day "X and me are best friends forever".

You and your DS should probably focus on developing a range of friendships with both boys and girls. That way when the inevitable falling outs happen he has other friends to play with.