Probably the battery in the security light was sized to be just enough for that purpose. It probably can't handle much more load, at least not for long. Also, the solar panel that comes with it will also be sized just enough to drive the light in whatever worst case scenario they designed for. Depending on the quality (and price) of the light, that scenario may vary greatly. For example, run for 3 full nights in the winter on 8 hours/day of heavy overcast, or maybe just at least 6 hours of night run time.
In either case, you will probably need your own or additional solar panel and battery. Does the shop light have to be a off the shelf AC unit? Can he get one more suited for the purpose that runs off of lower voltage DC?
In general, if you are looking for off the shelf solutions, I would look into 12V DC systems since there are various products available for use in cars. This includes DC to AC inverters up to 100W and more. That should be well more than enough to run a shop light.
