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She can't do anything on her own.
Literal
She [topic-は] alone [as-では] anything [emph-も] cannot-do.
一人では adds は to 一人で for contrastive emphasis — 'when alone, [as opposed to with help]' is the implicit contrast. The 何も + negative pattern produces 'nothing/not anything' — the universal-quantifier pronoun 何 picks up scope from the negative predicate. The same pattern works with 誰も + negative ('nobody'), どこも + negative ('nowhere'), 何にも (intensified 'nothing at all'). Whether this reads as a fond complaint or a sharp criticism depends entirely on tone.