。
She treated his broken leg.
Literal
She [topic-は] his [genitive-の] broken [past-した] leg [genitive-の] treatment [object-を] did.
骨折した足 ('a leg that broke a bone' = 'a broken leg') uses a past-tense verb as a prenominal modifier — common in Japanese for marking a state resulting from a past action. 骨折する is itself a 漢語 + する verb: 骨 ('bone') + 折 ('break'). 治療をする ('do treatment, administer treatment') is the periphrastic version of 治療する. Both are valid; the periphrastic form (with を) emphasizes the act of treating as a discrete thing.