Difficult situation, ghost, but I think a little lie saying that the cat needs a specially medicated diet and any other food will worsen her health condition and could she please stop feeding her this other food as you don;t want to have to forward all your vets' bills to her, but will be forced to, should the cat's condition worsen. If you want an actual condition that would be realistic for this situation then PM me and I'll tell you what we went through with one of our cats who is now on (very expensive) specialist food because of it.
And I just had another meltdown, dragged DH with me so we were both miserable, then I just thought fuck it, went and hugged him and told me to hug him back cos neither of us was going to go anywhere so we may as well just have a hug and forgive each other, which we did. He then gave me my anniversary present early and it is one of the most thoughtful and lovely presents I've ever been given - a leather-bound journal with paper pages made of cotton: a present to celebrate all three of our anniversaries. He said it was for when our baby was born, so I could continue the journal I've started. It;s just perfect, because I wanted to start a book of recipes, advice, pictures and memories - something for my baby to have if something happened to me ever. It's the most beautiful book - red embossed leather with creamy woven pages. I just burst into tears again. I must be a right fucking dream to live with at the moment! Oh well, at least I can laugh at myself.