Actually, the more I look at that the more curious I become.
The lower on an AR-15 is an unstressed part. There shouldn't have been any forces there strong enough to do that. Even if the buffer binds, there's not that much force once the bolt is unlocked. I've had buffer's bind up and they didn't damage anything. It just stopped the bolt carrier and the rifle failed to cycle.
Think about it, the only time there's any pressure at all pushing the bolt carrier back is when the bullet is between the gas port ant the muzzle. By design, the bullet has to have cleared the muzzle and the pressure dropped before the bolt unlocks or you'll have problems. Once the bolt is unlocked and the carrier is moving into the receiver extension the only force is inertia of the bolt carrier, and that's not much at all. I don't think that inertia is enough to crack that polymer like that. There's just not much force there at all.
In fact, about the only way I can see that break happening is if the bolt carrier caught the lower receiver at the vary beginning of it's travel. It would have to hit the lower before the bolt unlocks, the bullet clears the muzzle and the pressure drops. Then it would have several thousand PSI available (for a short time) and THAT could crack just about any lower. So somewhere in the first 1/4" or so of travel the bolt carrier bound on the lower. I'd bet it was the lower itself too, because if it bound in (or on) the receiver extension it should just strip the threads out and dump the extension. Which makes this whole thing very interesting because there's normally nothing there to bind on.
If I had to bet, I'd say the carrier tilted down and caught the threads in the lower, or went sideways and bound on the lower that way and that's what ripped the lower apart. I'd be very interested in seeing pictures of the carrier, receiver extension, upper receiver and the inside of that lower. I'd bet there's marks in there of the carrier moving around prior to this malfunction.
Either way the owner should send at least pictures back to Plum Crazy, so they know this happened and can try to figure out why. It might be a lower issue, or it might be some of the upper parts were out of spec, or some combination of both, but they can't fix it if they don't even know.