What to do if my friend has a condition called 'addison's disease' which is a fault with his adrenal glands. if im a match, would it be possible to donate?

Not possible: Addison's disease is effectively treated with hormone replacement (a glucocorticoid hormone and a mineralocorticoid hormone). No one has tried an adrenal transplant for this because taking the medication is not difficult and transplants of any type are expensive and risky.

Can be manageable: It's wonderful you'd offer to donate for transplantation, but Addison's is quite manageable nowadays. He needs to carry a cortisol needle (learn how and when you might need to use it), and if you're ever both in an accident and he needs to be treated, let his phyiscians know.