This girl should really disable anonymous comments as a first step. There's nothing shameful in it; she'd only be doing it to protect herself. The people sending her nice comments can do it just as well from their non-anon blogs.

Also, she can contact the Tumblr staff at the addresses Bark copied above, but I'm not sure staff intervenes in such cases. They certainly didn't when my own blog was targetted. (That was a couple years ago so things may have changed, who knows.)

Honestly, Tumblr is full of trolls and hateful idiots, it might be people from her school or just complete strangers who found a "target". Seen it happen before... it's one of the reasons I stopped using it.