So Redrum, are you saying, you or one of your colleagues has actually been and sniffed her chair?
There is a condition called Fish Odour Syndrome, where sufferers give off obnoxious odours, often smelling of rotten fish or meat, no matter how much they wash, the odour doesn't subside. Perhaps she has that.
If not, maybe she's just a dirty skanky bint, who has poor hygiene. Whatever the problem, your line manager should tackle the problem and tactfully discuss it with her. Things will only get worse as the weather gets warmer.