。
She tended to his wound.
Literal
She [topic-は] his [genitive-の] wound [object-を] treated.
手当する ('to treat (an injury), give first aid') is a noun-plus-する verb. The noun 手当 alone is heavily polysemous: 'pay, allowance, treatment, first aid' — context picks the right reading. Here, paired with 傷 ('wound, injury') and used as a verb, the medical sense locks in. 傷 is the general noun for any kind of wound — physical, but also extended emotional 心の傷 ('heart-wound').