There is a lot to consider when taking on side jobs. Obviously you want to
make it worth your while. Assuming that you have a truck, tools, and
insurance you need to calculate your overhead cost on an hourly basis.
Then
throw in your labor costs for yourself which should include social
security
payments. Figure out how many hours it will take to do the job and the
time
spent picking up materials and traveling to the job site. Figure out your
material cost. Add all of this up and you should be able to come up with
a
price that is profitable for you.

This is being done in his free time. My sense is that if he wasn't doing
this job he would be watching television.
Therefor it is based entirely on marginal cost. His insurance will cost the
same wether he is working or watching TV, so it is irrelevent.
His price should be something that is low enough to get the work, but high
enough to be a better use of his time than the TV.
You only figure in overhead when you are pretty well booked and only want
the best jobs. And even then you have to be very careful about it, for it
will lead to errors.