Here is your reply and it's very simple.
The 2nd amendment is simple.
A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed.
The issue is clear and the purpose is clear.
Short of changing the Bill of Rights this is the end of it. Short of amending the constitution, this is all there is.
And by the way, why do they need any new legislation. The National Firearms Act has a purpose perfectly in line with what these people claim they want to do. Why a new law or ban on magazine sizes, why not an amendment to the NFA? It's been amended before. Why not add these new features they want to the NFA descriptions for weapons. Shotguns already have a minimum overall length and a minimum barrel length, stock requirements, etc. and so do rifles.
Lastly tell him as long as he is busy reworking the 2nd amendment why not have another look at the others while he is at it because the purpose of the 2nd is to guarantee the rest.
He needs to stop worrying about inanimate objects and focus on what it is that people are doing these days that are making others do this crazy ****.