Really sad to hear this.
Atleast, she spent all her life with you and your family, making happy memories for all of you, while also enjoying the best company and care a cat could get.
I’m with Staci on checking her teeth, as, although I know very little about cats, it could be inflammation in the gums,
Still, as the vet themselves has said liver, and has taken care of her always, thus knowing her medical history, I doubt it would be something else. Still, very much sorry to hear. 😔😔
Hope whatever it is, she overcomes it, and becomes healthy once again.