It's much too harsh and will be ineffective. Punishments should also not be permanent especially with ADHD.
But I also have two children with ADHD and I know how devastating and soul destroying it is going through the phase where you are trying all the "normal" discipline and it doesn't work and veering into "I'm desperate I'll try anything". Extra-harsh punishments don't work either and are actually the opposite of what you should do, but I feel for mum, nobody hands you a manual (especially before diagnosis!) and she will likely be at the end of her tether, very upset by her DD's behaviour and the effect on your son, and probably worried about her turning into a bully etc. She is likely also spiralling that you will think she is an awful parent and/or DD will end up with no friends.
She is trying to do the right thing, even if it's not the right way to do it. We all make mistakes and hopefully as she goes further in her journey with learning about ADHD she'll find what support does work for her DD's behaviour.
I don't know if you can say anything which will be helpful, except to reassure her somehow that you don't think she's failing or her DD is a tyrant.
If you wanted to try, and DS wants to stay friends, you might say something like "Please don't worry about this; it will blow over for DS, I'll talk to him, no permanent harm done. I expect DD was feeling a bit exhausted after the week and it came out in her behaviour. We all say things we don't mean sometimes. Looking forward to the birthday party next week, let's plan our next playdate at a weekend so she's not so tired from school" - or whatever other future plan - basically reassuring her that you DON'T think she's a terrible parent or her DD is an awful child, that you do still value the friendship and it isn't necessarily this big huge thing she needs to stomp right away.
And if your DS is upset and doesn't want to play any more, you could say something instead about how perhaps they have just grown apart, but you can always get together without the kids. It's not that uncommon for girl/boy friendships to wane at this age anyway as they often become more aware of gender roles and gravitate more towards friends of the same sex.
Or maybe a message like "Hey X, just wanted to check in and say I hadn't realised DD was struggling so much at the moment, I really hope you get the answers you need with the assessment, do you want to meet for a coffee and a vent one morning before half term?"
If you do feel especially bad about the presents you could add something like "BTW, don't cancel her birthday on our account, totally respect you do what you need to do, but everyone makes mistakes - maybe she can earn them back."