"lost her footing" means that she slipped. It is a common and natural expression.
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hhtt Is it an idiom?I think it's borderline whether it should be called an "idiom" or not. However, it doesn't matter much.
hhttDoes it an idiom?I would call it just a fixed phrasing. Idioms (classically, at least) differ in meaning from their constituent words, but this does not: