If his house is small and well insulated, heat loss could less than 20,000 btu's.
I heat 3000 sqft with under 40,000 btu , outside temps 5C
15 100w bulbs generates 5200btu's, at 100% efficiency, which could result in a very noticeable difference. I am not considering the incandescent short bulb life here.
If the cost of electricity is equal or less than his primary heating fuel, he would be better to stay with the CFL's or LED's and add a electric space heater