It's got nothing to do with Glocks, no pistol can both feed a round into the chamber and have the extractor impact the primer simultaneously. And primers are not sensitive to rubbing, they discharge by an impact.
It's an extremely unlikely, maybe impossible, scenario to begin with (a extractor in any current production pistol contacting the primer while the slide is in battery) and then it gets doubly implausible because it relies on the primer itself behaving in a way that primers do not behave.
Basically, your friend could not have had his slide in battery and had the extractor contacting the primer at the same time, and the primer could not have been set off just from being contacted by the extractor, even with prolonged rubbing. For the extractor to contact the primer, the cartridge would have to be about halfway out of the chamber and angled in such a way that even the most casual glance would show you there is something wrong.
You can see what I mean with any pistol, doesn't have to be a Glock, all of them operating off the Browning short recoil system have pretty similar extractor/chamber relationships. Take a cartridge and touch the primer to the extractor, then look at how the cartridge must be angled in order to make it into the chamber. It just isn't something that a functional adult could possibly miss while loading their pistol and holstering it.
Manufacturers make subtle changes to parts all the time over the course of their product's lifespan, for major player's pistols it is almost always a reliability or durability fix, almost never a safety issue. An extractor redesign would be for improved reliability, maybe the change was to make the extractor function better as it's spring ages, but there's not really a conceivable way that an extractor redesign would impact safety.