Simple solution to stop this...
We Captains hold all the cards really.
REFUSE to fly with a paying passenger in the right seat and demand a replacement First Officer, one that IS employed by the company and recieves a salary.
If the airline refuses and threatens you, refuse to fly, go into the cabin. GET ON THE PA, and tell the situation to the passengers, stating that the Second in command will fly them their passengers to their holiday destination, and paying 50000 euro for the privelege. state that you can not morally, nor professionally support this,,and that due to this company procedure you have genuine safety concerns.
Get out.
Only a couple of times this must happen before the tabloids will be FULL of public outcries
Oh, yeah, you will probably have to find a new job, but if you fly for one of these slave companies that support PTF, you might be better of at home anyhow
At least you showed that you are worthy of your four stripes, have balls and take your responsibility towards our industry in general and your colegues Worldwide in particular.