Letters of Reference from co-workers

I think it this might be the most appropriate forum to post, but we'll see. :)

I had a worked with an RN who has since quit and relocated. He's in graduate school while working at a high level trauma center. A pretty impressive guy. He was my charge nurse and supervisor, and helped me tremendously when I first hit the floor as a new grad. I have hopes of continuing my education and experience, so I contacted him, asking if he would consider writing me a letter of reference that I can give to potential employers and tuck in envelopes with college applications. He said sure, as long as I wrote one for him as well.

To me, it's always been that those who write the letters of recommendation are the teachers, employers, basically someone "higher up on the food chain" than the person who's requested the letter. Can anyone give me any ideas as to what my focus in this reference letter should be, as I regard this man as an authority figure?