I read the other thread where you were clearly told the seat you are posting about was unsafe and could not be used. Why do you still want to use it? If the child's safety (or lack there of in that seat!) is not enough to convince you not to use that seat, perhaps you could think of it this way. Since you have been told in a very public forum that continuing to use that seat is unsafe and potentially fatal, not to mention illegal in many states with a proper use clause, you could potentially be held liable if you were in an accident and the boy was hurt. You could face criminal charges for negligence, endangerment or (depending on how severely he was injured) manslaughter. This is highly unlikely, but what is more likely since you are not the child's parent or guardian is that you could face a civil lawsuit and possibly be sued by the parents for knowingly endangering him.
If you cannot afford to buy him a seat and the parents will not lend you their backless boosters (or even better both the backless with the back they are not using) then you simply can't drive the child anywhere. Plenty of people without driver's licenses babysit kids and manage to survive it, so although it's not ideal, you can too. It's better than getting sued or having that weight on your conscience if something happened to him.