Oh dear, how awful for the other mum - and her reply :(
I do feel for you too - that gut wrenching awfulness when it's sent and you realise the full horror. Worst feeling, ugh.
I did similiar with an email - had let a nanny go after three weeks when it became clear she was utterly unsuited to looking after children in a home environment and it was a disaster. It was a senior position and i needed someone very experienced & therefore wages reflected that, and went through an insanely expebsive agency to recuit.
Anyway, she'd ended up doing some very stupid things in her panic at not being able to cope, and put my son in danger (previous she'd looked after very poorly children, more nursing than nannying really).
Anyway, she got quite angry with me, and said some rather nasty things - which I let her at the time as I know it's rubbish when you feel like you've failed and you end up hitting out at other people to avoid admitting it. It was a bad miss hire, and although she misrepresented herself at interview, the agency hadn't picked up on it, and nor had I.
Anyway my magnanimousness lasted to her face for the exit interview, then the next day I wrote up exactly what had gone wrong and all her faults, utterly no holes barred and I was very pissed off with her rudeness and the scare she'd given me. I wrote it to vent, before writing a constructive one to the agency which gave balanced feedback although made it clear she was unsuited to the role.
And I'd written both into email form, with no intention of anyone ever reading the ranty one, but i wanted to keep it so i could learn from the experience, so sent it to myself as a way id saving it... And I sodding sent it to her.
Was awful, all true but really really not what anyone should read about themselves, and she thought of course that I'd send it to someone else, and I can imagine it was soul destroying to read. I sent one explaining and reassuring her no one else would see it, but I have no idea if she believed me - tbh I wouldn't have. I still feel like a bitch thinking about it. Shudder.