I thought I'd ask if anyone had any ideas

My boyfriend's 16 year old cat is in trouble. She started losing fur a while back, then she started eating less and less and now it seems she won't eat at all, and of course she's drastically losing weight. They tried switching her food from the high quality dry food to some higher quality soft food and she only started throwing up everywhere. They took her to the vet a few times, got some blood work done and found out it's not hyperthyroidism, diabetes, and in fact they found nothing out of the ordinary.

Anyone have any ideas as to what could be causing this, or better what to do about it?