I am no authority on the labor law in Qatar, My understanding is
He is sponsored to be in Qatar to work for his employer and should not / cannot work anywhere else.
His free time after work is to relax and be fresh to work the next work day.
If he wants to do any side business, he has to get an NOC from his Sponsor and have it registered with the Authorities.
Legal action can be taken against him, and possible fine to the sponsor.
The above being said,
There are some people doing side business in the quiet.
He can provide his TRUSTED friends with the service knowing they will not tell on him. Both can get into trouble.