How should one "address" the person for whom I am writing a recommendation letter in the letter?
I have known John Doe for many years and am familiar with his work. We address each other on a first name basis. I've been asked to write a letter supporting John's application. When I write about him, should I write
Dr [or Mr, in case of undergraduates] Doe's works are well-written ...
or can I get away with
John's works are well-written ...
Personally I feel a bit strange writing Dr Doe for someone I know so well, but I wonder if it is better to err on the more formal side?
For what it is worth, the field is Mathematics. And for future reference, I do not want to limit the question to a particular level of application (for graduate school, fellowships, or for jobs); I suspect that shouldn't make a difference in the answer, but if it does, feel free to indicate.