LOR
Something along the lines of the following:
Dear (name of pilot recruiter):
I first became acquainted with Joe Blow when he was my flight student at Acme Aviation University. Joe was an excellent student and earned an "A" in Acme's instrument rating course. Later, Joe was hired at Acme as a flight instructor and after building flight experience at Acme was hired at Unsafe Air Freight, where we reunited. I was a Captain on the Beech 99 at the time and Joe was my first officer for many runs. Joe has since upgraded to Captain on the Beech 99 at Unsafe.
Having flown with Joe Blow first as my flight student and then as my First Officer, I can attest to his superior abilities as a pilot. Joe is a quick learner and works well with people. Above all, Joe Blow is a hard worker and loves aviation. I am confident that Joe Blow will be a fine addition to our company. I recommend him highly for hire as a Flight Officer.
Sincerely,
Gizbug
Something along those lines. Hope that helps a little.