I guess this quick ("in black") profit makes people blind.
if he does it legally he needs to pay taxes on his profit and be a real entrepreneur with quarterly paperwork etc.
Substract your gas and other costs and there's only one winner... Uber.
I don't think there's even a legal status for such work in Belgium so people who drive for Uber are sticking their neck out and all possible fines will be for them.
And no social contributions from Uber's side either. so they're safe on the other side of the pond while the driver is taking all the risks.