What tax form is this business being reported?

What type of vehicle is it?  I'm picture one of those 'box trucks', and 100% business would seem reasonable.  But then you said "hauling trailer containing bread", so it may not be what I'm picturing.

If it is a just a 'regular' automobile, then the client needs to have good substantiation of the business miles.  Once you have the number of business miles, contrast that to the total odometer miles during the year and you'll have your answer to the business percentage.