。
She's making money hand over fist.
Literal
She [topic-は] rapidly money [object-を] is-earning.
どんどん is one of Japanese's most common 擬態語 (state mimetics), conveying steady, rapid, unstoppable progression — things happening fast and continuously. It can describe accumulation (どんどん貯まる, 'pile up steadily'), advance (どんどん進む, 'progress rapidly'), or improvement (どんどんよくなる, 'get better and better'). Paired with お金をもうけている ('is making money') it gives the picture of a steady stream of profit. もうける (儲ける) is the standard verb for making/earning money in a profitable sense, as opposed to 稼ぐ ('earn through work') which is more about wages or labour income.