My old fella was a chartered accountant and he always told he when employing subcontractors you need to do two things to prove they're not employees: 1) make sure they're doing the same type of work for someone else and 2) make sure they have the right of substitution ie they can send someone else of comparable skill/ability/clout to do the job. If you don't do this you run the risk of them being classed as employees which is fair enough.
The bottom line is, these guys all earn a fortune, they're grown men and can pay a tax advisor or accountant to tell them what their sitiatiom and liability is.
In short, it's his own stupid bloody fault if he gets done.