The wheel does not need to spin to install an axle properly. You can do axle replacements with a normal jack. My bet: mechanic did not properly torque the bolt for the axle. It's quite a hefty torque spec, and due to not having the BMW repair manual the mechanic just gave it a couple of hits with an impact and called it tight enough rather than properly torquing it. Due to direction of spin of the wheels, the bolt then loosened on its own since it did not have the proper torque (this is why Porsche center locks on the passenger side are lefty tighty righty loosey). That then introduced play in the splines, and every time your brother accelerated the axle's wheel-side splines essentially hammered against the splines of the suspension carrier. Axles are not designed for this kind of shock forces and and eventually the shock fatigued the axle enough to just snap.
I would look into replacing the entire suspension carrier to be honest, since if my theory is correct, the carrier's splines are likely damaged as well due to this hammering.
Also as a quick aside, GKN is not OEM. It is OE, a step above OEM. OEM means they directly make parts for BMW, but not necessarily the literal part. OE is the exact same part that would have been used by BMW during assembly, but with the BMW logo removed. In terms of replacement parts hierarchy (and thus price tbh), it goes:
- Genuine
- OE
- OEM
- Aftermarket copies