If he has his mind set on it, just let him go and make his own mistakes.
Popups really aren't much, if any, lighter than hardsides of similar size and with similar features.
There really isn't anything out there that won't grossly overload a Tacoma once loaded and ready to camp. It will be over by hundreds to as much as several hundred pounds, and it will need helper springs or airbags to hold up the rear end. It's entirely up to him if he's comfortable with that. Bad things can happen but probably won't. The more imminent concern is will he be happy with his choices, or will he have just wasted thousands of hard-earned dollars.