https://my.chevrolet.com/home/?evar18=ch_header_link
Have your friend create an account. It may tell him if his warranty is in tact. I'm thinking there is a chance that the dealership may not have actually flagged it, maybe they did. It is possible that part of the diagnosis for a stumble involves checking the PCV system, which they cannot do with a catch can added to the system. If that's the case, they might have just said no without flagging the denial. (not to mention that if there is an issue with the can or the installation, it could actually be causing the stumble) Adding a catch can is changing the system as designed which is the system as warrantied, and that can cost you the warranty. You can fight it all day long, but this is one you will lose in the end. Don't be a victim of mis reading the MMA.
The other thing for them to try is to remove the catch can, and go to a different dealer. The worst thing that can happen is they tell him the car is flagged and he'll know for sure then.
Ladies and gentlemen, if warranty is important to you, be very careful about the modifications you do. Don't do anything without talking to a service manager first. This is especially true during the first year of ownership. It's typically in this first year that the factory installed gremlins show their faces. Do yourself a favor, ask questions and wait.