。
She used up a whole bar of soap.
Literal
She [topic-は] soap [object-を] one-piece used-up.
一個 (いっこ) is the counter 個 for small discrete objects — here, one bar of soap. The ~てしまった ending marks completion with a sense of 'went through the whole thing' — she used up the entire bar. 石鹸 (せっけん) is 'soap,' a word with notably complex kanji that's often written in hiragana instead.