It is caused by the diet you have fed it. Another instance of a high fat diet and the seeds.
I can't say enough about the way all of you feed your birds and what it does to your birds. I have studied these birds in the wild and they DO NOT eat seeds in the wild. They eat insects as a main staple which are high in protein.
Seeds cause a disease known as Fatty Liver Disease. A parakeet on a seed diet will live about 4-6 yrs. Just long enough for you to get the bird just the way you want it, and then it dies. Pet store lining their own pockets with your money. If more people would use their common sense, more birds would be living longer and healthier lives and saving their money.
Start feeding your keet veggies. It might be to late for your bird, or it may not be. Educate yourself now on the diets of your birds. Don't rely on a pet store for your education. They are only out to take money out of your hands.
I commend you for taking your keet to two vets. Where most won't take a keet to one.
Depending on how involved the tumor is, it can be removed. But at what cost? With the keets age, would it be wise? Evaluate the situation as best as you can for now. I'm sure you will know when the time is right.
Good Luck!