I don't think there is any traditional way of curing flu for cats my friend.
You need to bring it to the vet soonest possible to avoid the flu from becoming worse.
If you are staying around Cheras there is a pet shop that sells flu pills for cats and its very effective 
It only cost RM3.00 per pill.

Tips on fattening a cat.... give it boiled chicken meat 3 times a week.  Make sure that the chicken meat is fresh and contains meat and muscle.  Boil it in water for a few minutes and then tear the chicken meat into small pieces and feed it to your cat.  You can also give your cat food high in protein e.g. Innova Cat Food which contains very high Protein.  Another way is get some cat supplement e.g. Nutrical which contains vitamin B complex which will enhance the cats appetite.  Good Luck!