B9exchange
I think she is more likely to be upset if you refuse to come to her house ever again without an explanation. Be honest, explain that the smell of cat food makes you nauseous, and what a struggle you had last time. Add that you certainly value her as a friend, and hope to see her again soon, either at your place, or at hers with the food out of the way!
This is to my mind the obvious solution.
I do wonder what she is actually feeding her cats, though.
In Denmark pet food doesn't smell, nor did the cat food I bought our cats when we were sailing in Germany, Belgium, France and the Netherlands.
I was buying brands that are commonly available so I imagine they are the same ones you can buy in the UK.
The only way cat food would smell so strongly is if it is standing about near a radiator or in sunshine for far too long, or if the dishes are not properly washed after use.
Few cats would eat food that smelled rank, as they are hunters by nature, not scavengers. Of course, these four have had to accustom themselves to it, I suppose.
Be frank with your friend and tell her you cannot bear the smell of cat food. You can make the statement sound like a personal quirk, rather than a criticism of her housekeeping.