Mayer is a big name, but people are always moving around, or thinking about moving around, particularly in big cities with major programs. These sort of people are in high demand.
One individual does not an institution make. I would highly recommend against deciding on a program because of one person there. I do recognize that it's hard to maintain this sort of perspective when you're just getting started in the field, but Columbia is a great program, with or without specific faculty members there.