You could kill all the other breakers in his panel, turn all the lights on, and have some kid open & close the garage door as much as he can for an hour and measure it on the meter or something. :-)
Or you can install only high-efficiency lighting and put an occupancy-sensor switch on it. The biggest cost to him would be if someone left the lights on, since the motor is probably less than 200 watts, and only runs a minute or two per day. 200 watts is 0.2 kw, 2 minutes is 0.033 hours, so...
0.2 kilowatts x 0.033 hours is 0.007 kwh/day x 365= 2.5kwh/year
A couple of 40 watt lights is 0.08 kilowatts, but if you left them on for a whole day (24 hours), that's about 1.9 kwh per DAY.
With an occupancy sensor or timer switch that can't happen, and odds are it would add up to less than 1 hour per day, so it would be at most the power rating of the lights, x 365 days/year.
Or you could just comp him a 6-pack once or twice a year for being such a good sport. ;-)
If he insists on the penny-accounting, look up the specs for the actual power draw of the garage door opener, measure how many seconds it take to both open and close it, take a reasonable stab on the average daily open/close, and add it up for a year in hours x kilowatts. Do a similar estimate on garage light use & power. It's not nothing, but it's probably less that the value of the time it takes to calculate it.