。
She's just acting on instinct.
Literal
She [topic-は] instinct [possessive-の] as is-acting [assertion-んですよ].
本能のままに振る舞っている — のままに ('as is, according to') means following something without restraint. 本能 (ほんのう, 'instinct') is the driving force. んですよ adds explanatory assertion — 'the thing is, she's acting on instinct.'