Maybe your "friend" is giving you mate's rate to cover his increased insurance payment like hef mentioned. If your friend has any business sense he will still be making money while expecting some form of loyalty for "helping" you out (usually over Christmas and various other public holidays.....). In my opinion this will be the only "hidden cost" involved.
You are obviously aware of the dangers of doing "business" with friends and family?
He's not working so the 'don't work for free' argument doesn't stand up.
That is only true up to the point where the CPL is obtained. Up to that point I fully agree, go for it! If you have the money to make yourself more marketable, do it!