。
She's on a diet to lose weight.
Literal
[topic-は] become-thin [in-order-to-ために] diet [object-を] is-doing.
ダイエット, borrowed from English 'diet,' has narrowed in Japanese to mean specifically a weight-loss regimen — the broader English sense of 'diet' as 'what one eats' is generally rendered with 食事. Pairing ダイエット with をする gives the verb form 'go on a diet' or 'be dieting.' The polite ~しています keeps the register friendly and conversational.