I just ran some very rough, very conservative tax numbers...including regular income tax, state income tax, social security and medicare taxes. Adding in the 400.00 a month that would give him approximately 79945.00 net a year. Allowing for health insurance that might change to somewhere around 73945.00. That's 6162.00 per month in disposable income.
So yes, I agree that a mortgage of 2100.00 is affordable at that level. The only reasons I can think of that might make his budget tighter than it should be is if he is putting a whole lot into a 401k, which he probably cannot continue to do, or if he is way overwithholding and getting big refunds every year. Of course, his tax situation is going to change once he gets divorced, so perhaps any overwithholding should be left alone.