I don't have a whole lot of experience with guppies because I have trouble getting healthy stock.
But even in my limited experience, I have seen female guppies show dominant, harassment behavior towards other females. I've never seen one take a chunk out of another, but I've certainly seen the behaviour that could lead to that.
Guppies (no matter their sex) can become aggressive towards another despite of the fact that they are considered being friendly fish. But the cut in her tail looks like a very clean cut. She could've also damaged herself at something (wood or something else) by accident.