I agree - if she's working hard on everything else, the problem is not her ethic, it's the class. It's important to fix the thing that is actually broken.
If it's impossible to change her class, can you do anything about her workload in that class? For each new worksheet topic, can she be required to do just enough to show mastery, and no more? I've never had any luck getting differentiated work, but I have often been able to just get work eliminated. Ideally, could she maybe even submit AoPS problems (that you find) on similar topics in lieu of the more basic questions she's getting at school?
I've heard an incredibly huge amount over the years about "he's got to learn responsibility"; "he's got to learn to just buckle down do the work"; "he's got to learn that in the real world we don't just get to do the parts we like". And I have seen huge damage caused by these well-meaning but completely clueless views. What kids actually learn? To hate math. To believe that their learning needs are illegitimate. To not respect their teachers. To see education as a waste of time. And for some kids, to start building up huge anxiety about such classes and work, leading to shutting down, acting up, or depression. Sorry to sound a little over-the-top here, but I've lived way too much of this, and it's not benign.
One of the best things I ever heard a parent say was "I just want my kid to work as hard, and as long, as everyone else in the class." Now THAT is teaching work ethic.
