I would not put a heat buld in a recessed can. Too much heat builup could cause a fire in the
ceiling, and the fixture may not be rated for the wattage you'd need.
Check the vent. If it vents into the attic that could cause cause mold or rot your roof sheathing.
For a renter, I would wire the fan to the light, so that you know that the fan is always
being turned on as long as he doesn't shower in the dark.
Re the cold bathroom:
See if air is actually coming out of the vents. If it is not clogged, but just does not have much
air, possibly a booster fan may be needed.
You could go with a heat bulb in a new non-recessed fixture, or maybe someting like a
Radiant Ceiling Panel which has the virtue of being extremely thin.
See, for example:http://www.prosupplyco.com/cp123-375w12 ... MgodWhEAOQ