Many faults like this are caused by some behaviour of your friend that none of Apples internal testers did.
For example, maybe your friend used a specific app that no tester every used.
I remember once a big company doing an emergency product recall because they hadn't tested putting the product in fleece lined pockets. If you put it in fleece lined pockets, it wouldn't boot anymore due to the static discharge of the fleece rubbing on the plastic surface as you walked around.
Yet they had 1000+ beta testers, yet were somehow unlucky that not a single one used fleece lined pockets!
For example, maybe your friend used a specific app that no tester every used.
I remember once a big company doing an emergency product recall because they hadn't tested putting the product in fleece lined pockets. If you put it in fleece lined pockets, it wouldn't boot anymore due to the static discharge of the fleece rubbing on the plastic surface as you walked around.
Yet they had 1000+ beta testers, yet were somehow unlucky that not a single one used fleece lined pockets!