My 9 yr old cat is quite sick and is not eating?

she has hcm and a heart murmur and low blood pressure this for some reason (the vet cannot quite find out why) has affected her eating. She has always been a fussy eater i only offer her cat food and she has never eaten human food however recently she just wont eat anything at all she normally has whiskers dry with felix as good as it looks wet food but now shes not eating anything much at all but when she does it tends to be the dry food and then is sick i have tried feeding her human food like chicken and tuna and salmon and things just to get her eating but she just refuses i went to the pet shop and asked them for some advice and they have advised me to feed her kitten food as it has a high protein and calorie intake and as she is soooo thin it wud be a gud comprimise i have brought it and gave her 1 pouch and she ate the lot which is a first is this really a good idea or am i doing long term damage ??? btw i havent taken her back to the vet as their veiw was to put her to sleep when hcm was discovered but i said no cos she is not in pain.