。
She certainly behaved quite foolishly.
Literal
She [topic-は] considerably foolish behavior [object-を] did [reflective-ものだ].
~ものだ has multiple uses; here it's the reflective/exclamatory one — looking back on a past event with feeling, often nostalgia, surprise, or criticism. 'She really did do something quite foolish.' Other uses of ものだ include the general-truth or expected-behavior reading (人は変わるものだ 'people change') and the wish/desire reading (帰りたいものだ 'I'd love to go home'). ずいぶん ('considerably / quite a bit') is a moderately strong intensifier — softer than 非常に ('extremely') but stronger than ちょっと ('a bit'). 振る舞い ('behavior, conduct') is the noun form of 振る舞う ('to behave / act').