。
She did it entirely of her own accord.
Literal
She [topic-は] that [object-を] [completely-まったく] [voluntarily-自発的に] did.
自発的 (jihatsu-teki) is a Sino-Japanese な-adjective meaning 'spontaneous, voluntary, of one's own accord' — the kind of action driven from within without external prompting. Its adverbial form 自発的に modifies the verb. まったく ('completely, entirely') intensifies the claim, ruling out any partial coercion or hint. The whole sentence reads as a pointed assertion that no one made her do it. Compare with 自分から ('on her own initiative') and 自ら (literary 'oneself, personally') — all three orbit the same self-driven-action concept with different registers and shadings.