Translation:Her mass is approximately sixty kilograms.
This does not have to be "says", it could be a written sentence. It could be from a scientific article, for example (especially in physics, chemistry or engineering). Also, we do not know it is about people. It could be a boat (though one uses "weight" for them in the common language as well).
I wanted to avoid that so I offered a boat as an example. But yes, if it is a physics exercise and we are speaking about a sphere (koule) or some other object, then we would use "its", so perhaps we should accept it.
I will just add that here hmotnost is the more "scientific" term in comparison to "váha", which is colloquial. That's why the author of the sentence may have preferred "mass" instead of "weight".