There's a WHOLE lot of assumptions being tossed about here. Pretty disappointing. Has recent history not taught us to maybe wait until all of the facts come out before we throw out theories?
In this case, facts are the facts.
I'm not familiar with Michigan law, but in most jurisdictions (every county in Ohio) when you assume control over the vehicle, it's your responsibility to check the vehicle. You assume possession for everything in the car.
It's a felony to be in possession of an unregistered firearm.
It's certainly plausible he just has terrible friends, but that doesn't make him less responsible for what he's being charged.
It's all quite disappointing.