Any advice on how to proceed with this issue?

A friend has a 2004 Chevy 3500 passenger van with dual A/C. The vehicle in service date is less than 3 years old but it has around 45k+/- miles so it is out of warranty. He noticed that the A/C wasn't working the other day, took it to the dealer and was told that there is an O-ring leaking at some fitting and the rear A/C line, where it goes through a rubber block (?) corroded through (or something like that). They're going to remove/clean and install a new O-ring and then splice in a tube for the corroded part, instead of replacing the line. All this for a mere $540. The dealer said it's a common problem in the Northeast. He's having them fix it and asked for them to document everything. A 2004 with corroded leaky lines and a bad O-ring?!?!?! Doesn't seem right. My friend doesn't expect GM to cover everything, but some help would be nice.

Any advice on whether or not GM will help out?

GM will help out if you are within the 3 years milage needs to be under 50k AND it is up to the service manager. So if he is a good customer, it can possibly be covered. It is all their call.

A/C used to be covered by mileage only because of the seasonal usage, read warranty brochure (if still have it) and give GM a call... never hurts to try.

Yes it is fairly common up north for the lines to corrode, mix the salt with whatever else may be on the road and it all corrodes. Lines are aluminum.