sometimes there is nothing that you can do. try feeding cooked ans pealed green peas. i just used canned peas ans pinch off the skin. sometimes they can get constipated and get a bubble in their gut. i have a pearlscale that occasionally gets air in its gut but it is usually stuck at the top of the tank upside down. the peas always seam to do the trick. worth a try, they are good for the fish even if it does not help.

also don't feed floating food. buy sinking pellets.

First off..Congratulations on 7 years. That is a feat for most goldfish. Most don't make it a year. I had mine for 37 years..He was 17 1/2 inches long..He was my baby.. I had a similar problem. I too used the canned peas, just make sure the skin is off. It should work, if not, call your local vet office, they will give you a number of a person who can tell you exactly what to do. Also..As your goldfish ages, you will need to do things differently with your tank. Usually fish over 10 get stressed out when you do water changes. So only remove 1/4 of the tanks water once a week. That way there is no stress. It will need to be changed once a week..lol especially has your friend gets bigger. I ended up putting mine in a 100 Gal tank. I added other goldfish with him, but he ended up eating most of the fish I put in there..Here I found out a bit later, he would only eat the Male Goldfish,(they were competition..lol) But he left the females alone..Go figure. You can train your goldfish easily too. Good luck with your Friend..I hope all goes well, Keep us posted!
Cynthia