。
She couldn't look him in the face.
Literal
She [topic-は] his [genitive-の] face [object-を] directly see thing [subject-が] couldn't.
まともに ('directly, straight on, properly') is a useful adverb for 'face-to-face' or 'head-on' interactions — まともに見る ('look directly'), まともに話す ('talk straight'), まともに考える ('think seriously'). The whole phrase implies emotional weight: she couldn't bring herself to make direct eye contact, perhaps from shame, guilt, or grief.