"His boy weighs 57 kilograms" is correct. Also, "His boy weighs 50 7 kilograms" is an extremely unnatural answer and should be removed.
I put 57 just now and failed
Why not kilogrammes too?
..or about 125ish pounds.
"Her boy weighs fifty seven kilograms" is not accepted.
Pehat: I think your translation would be: "Băiatul ei cântărește cincizeci și șapte de kilograme." (Her boy weighs fifty-seven kilograms.")