Ignore the people who tell her to get a thicker skin or build up resistance to toxicity. It doesn't work that way for everyone. I've been playing for over a year and I certainly haven't built up resistance. I doubt I ever will, either, for it's not who I am, and I don't feel like I have to deal with people's toxicity every time I play. "It's part of the game", my butt. It shouldn't be. It's why I generally mute everyone at the start of the match myself.
It's what your gf should do, too. Just use pings to receive and send messages. Or if you must, you can type without needing a response. Just a simple "help me" or "retreat". No shame in that at all. It certainly works for me.
I know it ain't the best teamwork, but random people usually aren't the best at teamwork anyway. It's not ranked, not a ranked team, and not friends, so whatever. Those very same toxic players would probably consider it "bad behaviour", but if you ask me, I'd rather win a game with minimal communication than lose a game because some dick in my team can't stop spitting toxic goo at me and others. And since you can't control the as shatter of others, take preemptive measures and mute them, as that is within your control.
Just my two gold pieces.