This Andel is on the downside of a very long life and to be honest you should just give her whatever she likes best. No new diet will stop the coughing. I've been there and I know what you mean. I have had my own dear Maltese's (2) of them suffer the same fate. I also foster old and injured animals and the best thing you can do is make them as comfortable and HAPPY as possible. If shes eating fine etc. let her alone. She's too old now to irritate with any changes. Give her all the things she loves and make her comfortable and happy for whatever time she has left. There may be some medication she can take for her cough but NOTHING will stop it.
She has had a long and apparently happy life with you. Be strong and if the coughing gets too bad and she starts to fail or seems in pain BE STRONG for her and let her go peacefully.