I was trusting her as shes generally very sensible
Meetme, with the greatest courtesy - she is TEN YEARS OLD.
Please do not trust her with internet access.
Your faith in her is touching, but look at the fallout from her chatting to a boy her own age, on a medium you cannot police because snapchat deletes so I cant see those.
You say she doesn't have access to any other SM and it isn't unfettered. Don't be too sure of that. How do you know what else she has deleted, what other sites her schoolmates are excited about, what she may even have viewed on a friend's similarly unpoliced phone?
The fallout from her class snapchat is ... unpleasant to have to deal with, but entirely manageable.
The fallout, should she venture onto other sites, & believe that she knows who she is chatting with, could be immense.
I hope you are able to use this as a teaching moment for DD, & that you are able to focus on rebuilding bridges with your confident friend. The friend may even have some tips on how she polices her son's phone usage - so use this opportunity to forget about feeling upset that she dealt with this as a parent first, & friend second.
Apologies from DD, contrition & common sense from you, & no doubt your confident friend will appreciate your reaction this time, because her saying believe me this is not how Bob (the boys Dad) wanted to deal with this doesn't sound like a threat to me, it sounds like an affronted reaction to you focusing on your friendship with her, rather than the upset to her son.
So in your friend's view, her response was telling you that this IS her dealing with it in as friendly a manner as possible under the circumstances. Take it on the chin, educate DD, & emulate your friend's confidence & assertion - it'll all come out in the wash so long as you can ensure DD understands why what she did was wrong.