。
She testified against him.
Literal
She [topic-は] him [against-に対して] unfavorable [adj-な] testimony [object-を] did.
~に対して ('against, with respect to') in a legal context, pairing with 不利な ('unfavorable, disadvantageous') and 証言 ('testimony'). 不利 uses the productive 不- prefix that negates Sino-Japanese roots: 不親切, 不可能, 不便. 証言をする is the periphrastic verb form; 証言する also exists. Together: legalistic phrasing for 'gave testimony unfavorable to him.'