Could this mean both her own weight, and/or that she is placing sixty kilograms of something on a scale and weighing it?
Grammatically both, but semantically, would it make any sense? It's still some 60 kg object, and it's rather the object that should be put in the sentence then...
But "Ona waży krowę" (she is weighing a cow) is a valid sentence, the grammar does not change (Accusative needed).
Thanks for always having such helpful answers! Really I was just wondering about the possible meanings of the verb, the sentence is quite clear as you said :)
I'm actually quite surprised with the Polish language here.
"To weigh" is also ambiguous in English, of course, but for some reason I thought Polish would make a point of using one of those reflexive "się" constructions. Oh,well. :)
Reflexive "ważyć się" is used when you step on a bathroom scale to check how bad it is ;)