。
She's on a diet.
Literal
She [topic-は] dietary-therapy [object-を] is-doing.
食事療法 (しょくじりょうほう) literally means 'meal therapy' — it covers both medical dietary restrictions and weight-loss diets. The をしている pattern frames it as an ongoing practice. In casual speech, ダイエット is the common loanword for dieting.