Alright, info.
I don't think his brother broke anything. He just pinpointed the issue by altering the seal of the sunroof with the car's roof.
With that, two things:
1) The G8's sunroof module - by which I mean the entire, installed assembly - has a drain tray to catch rainwater that makes it through the seal and funnel it out of the car through drain hoses. This is because the sunroof seal is not perfectly tight, anyway. To do that, it'd have to overlap with the roof, which wouldn't look quite so neat. So as a backup - and also likely to cover themselves against lawsuits if an owner left the roof open - there's a catch tray that surrounds the sunroof and drains outside the car.
Proof? Here's a diagram of the sunroof module from the factory service manual.
So your car's not going to become a mildew-ridden bucket in the meantime.
2) While the sunroof CAN be physically adjusted, it's as likely in this case that your sunroof control module simply didn't fully close the roof. It may need to be re-timed.
You see, the sunroof control modules in our cars don't work by sensors that pick up where the roof is. Instead, when your car leaves the factory, it's run through an initialization process. This process allows the sunroof control module to figure out how long it takes to open your sunroof, specifically to the full vent position, and then extrapolates from that time value to figure out how long it takes to move the sunroof to any position.
Luckily, you can re-run that initialization procedure yourself. Here it is below from the factory service manual. Give this a go before taking the car in to the dealer.