Hey, OP. I'm kinda in the same boat as you - my daughter is around the same age as yours, and loves playing this game. However, one caveat: I do not allow her to enter any group content without either me or her dad (or on the best days, both of us) "in person". That way she has a shield against idiots. We also regularly discuss online behaviours with her, because at her age, she doesn't have all the coping skills an adult has to deal with it, let alone the understanding. We've talked about bullying, both online and off, as well. I imagine you've done the same, but it seems like it's a good time to go over all that again with your kid. Remind her she's awesome for doing this stuff, remind her just how far she's come as a player, and she doesn't need to give up because of a setback. Help her understand mistakes happen (I reckon you've already done this, but it bears repeating for anyone else who finds themselves in a similar situation), and the best way to get over it, is to get back up on the horse and try again.

Only this time, please go with her. It's both more fun when you do stuff like this with your kid (as you already know!), but it's for her safety too. Also, you get the chance to be the best parent by telling a bully to get stuffed and what-have-you if the situation arises. You will show by direct example how to deal with bullies, probably some of the best teaching opportunities you'll get for that sort of thing (let's make a negative experience have a positive outcome at least). On top of that, it's a great, subtle way to work on communication and teamwork with your kid, because you can discuss strategy and whatnot as you go.

Chin up to your kid, and again, please remind her how awesome she's been, and that mistakes happen - don't miss the opportunity to learn from them. <3 & *hugs* for your daughter!