replace all 4 tires when 2 go flat? '89 cherokee

hey guys-so my cousin runs over a wheel/tire that some jackass drops on the
freeway. going about 80 mph, he hits it and flattens both left side tires.
got towed to Pep boys (closest place) and gets told he need to change all 4
tires because he has a 4wd car and only changing 2 could damage his
drivetrain. what do u think about this?

i say BS. shouldn't the differentials handle the slightly different tire
sizes ok (new vs old) and in the snow, the tire slipping on the snow would
take care of that too right? he's in colorado (boulder) so snow is a
concern.

they are trying to sell him a pep boy-brand tire or something else that
costs a whole lot more ($95 pepboy tire vs $150 other brand (can't remember
which). no, they don't carry bfg's

thanks!