"Elle a oublié son imperméable."
Translation:She has forgotten her raincoat.
imperméable is both and adjective (meaning waterproof) an a noun meaning "raincoat".
Yes, and my point is that "waterproof" is also both an adjective and a noun in English with exactly corresponding meanings. (At least in UK English.)
Personally I never say "raincoat" and prefer to say "waterproof" or "waterproof jacket" instead.
An old fashioned alternative might also be "rain mac" or "plastic mac", short for mackintosh.
'I am wearing waterproofs', to denote multiple waterproof items, is also common.
I used to wear waterproofs frequently during my Vespa phase.
Sorry, mistook your post for a quasi-question. I'm Australian and, although we don't use the term "mac" we know what it is. :-)