Need Advice

I have a 7-year-old tabby. She's a sweet girl. However, she has lost dramatic amount of weight. We have taken her to the vet, but they haven't found anything, except some signs of stress. We have been doing everything, given her chicken, we tried baby food (on vet's suggestion), even tried the gravy. She is still throwing up foam, and she is barely eating. She is drinking water. She does give us cuddles and not reclose. We got her a calming collar per vet's additional suggestion to ease the stress. we have a room for her where she can escape our 10-month-old puppy. It has food in there, water, and a litterbox.

I just don't know what to do anymore. I don't want to lose her. She is so young. Has anyone seen this? If anyone could offer some sort of advice. I would appreciate it.