。
She delivered a slap to his face.
Literal
She [topic-は] his [genitive-の] face [to-に] flat-hand-strike [object-を] made-eat.
An unusual idiomatic construction. 食わせる ('feed, make eat') is used here in its 'inflict, deliver, deal' sense — Xを食わせる ('inflict X on someone'). Works for blows (一発食わせる, 'land one'), tricks (一杯食わせる, 'pull one over'), and other things you 'serve up.' 平手打ち ('open-handed slap') is the formal noun for a slap, distinct from the colloquial verb ひっぱたく.