Since it is your best friend and best friends are usually so unique to each other you would have to say something about what it means to you. Say something about the impact they made on you when they were not retired and as a person what they mean to you now. Talk about all the time you both will have to spend with each other now your friend is retired. Talk about the things you will do together since you will have the time, like the vacations you can take now or hobbies you have always wanted to pick up together. There are many things you can say but make sure what you say means something to the both of you.
