Cats can be very social creatures and often form deep attachments to members of their family, whether human, feline or other animal. It is normal for cats to grieve when a loved one dies, and they can show this grief in many different ways. Weight loss and vocalizing can also be symptoms of an underlying medical issue, especially considering the age of your kitty. She should be seen by a veterinarian for a complete physical exam, lab work and blood pressure measurement. A hypoallergenic food shouldn't cause weight loss unless she is not eating enough. If she doesn't like the food, your veterinarian can recommend another one.