How should a student address a professor in a letter of recommendation?
I wouldn't worry a lot about it.  The worst that can happen is that part of the letter sounds a little off, but I'm sure promotion committees see much worse in letters from students, and understand it doesn't reflect on the professionalism of the professor.

If I had to chose, though, my personal inclination would be to write "Professor X" in the letter, rather than the professor's first name.  It's much easier to go wrong in the direction of being too informal, rather than being too formal.