Since this kitty is 12 years old, my first question would be, did the vet do a very thorough oral exam?
Cats at that age (and much earlier too) can have dental problems (bad teeth - cracked or broken teeth, cavities, an abscessed tooth, etc - and gum disease) that make eating uncomfortable or quite painful. With cats such a problem is not always easily noticeable, sometimes fussy eating and weight loss are the only signs.
Also, something can be going on in the digestive system and blood results don't always show such problems. It would be wise to ask for an abdominal X-ray and depending on what this shows, an ultrasound after that.
Your friend should also ask for a chest X-ray.
If the vet didn't do a thyroid test, your friend should ask for a thyroid test as well.
You don't mention whether there is any sneezing, nasal discharge, etc, any vague signs that might indicate a problem with the nose or the sinuses. These problems can affect a cat's sense of smell and lead to anorexia.
What about the stools? Does this kitty have perfectly normal stools and is she regular? Has the possibility of constipation been ruled out?
Please ask your friend for details about the stools. (Loose, hard, whatever, there may be a clue somewhere.)
I'm trying to say that at this point, anything that hasn't been looked at has to be looked at, and your friend has to insist on more diagnostic work, including the very thorough oral exam I mentioned on top.
she has been to the vet and had all the necessary blood tests and she is perfectly healthy
Unfortunately there are many health problems that don't show up in bloodwork results. Also, you need to find out what exactly was done.
Quite often vets only do a partial profile which doesn't cover everything.
In addition, owners can be given the wrong information about test results. (I remember one owner who was told nothing was wrong and sadly it turned out that the cat had liver disease.)
Here is some very basic information for your friend:
http://www.petplace.com/cats/anorexia-i ... page1.aspx
http://www.burns-pet-nutrition.co.uk/Ar ... isease.htm