Damn, that's really unfortunate. My friend had his iPhone taken in the middle of the day. The guy asked if he could make a call, then walked out of the store, and when my friend followed him, he said he had a knife and to walk the other way.
I understand where you're coming from with this mod, but in all honesty, the best way to prevent getting into a situation like this is to be smart. Of course I don't know the full story, but my advice to anyone is keep your phone concealed whenever you can and be mindful of your surroundings. May seem paranoid (Hey, I'm kinda tall for my age but I'm also pretty skinny), but I'm always watching around me for anything/one suspicious - especially when walking down a dark/sketchy side street.
Glad your brothers safe though. Situations like this can turn ugly fast.