The info he received from the instructor was bad, and could not only land him in legal trouble in mass, but could cause him to become federally disqualified from ever owning a firearm again. Though WRT the LEO telling him he can go to CT and buy post bans, yes he can. BUT they MUST be kept out of state. I could buy as many as I wanted so long as they didn't cross the border.
This is 110% untrue. No where in mass MGL does it say that pre ban mags can only be used in pre ban guns. That sounds like some gun shop lawyering being passed onto the internet. If I go and get my hands on a brand new AR-15 that is AWB compliant, and buy a mag made in 1991 I am legal to put it into what ever gun I wish.
Mass gun laws are backwards, vague, arbitrary, confusing, contradicting and (I believe) intentionally so. I use a diffrent forum most of the time because this one is blocked at work. Its a regional site, and the majority of the members are from MA. Even there we get confused. It is extremely common to ask someone to site the law they are referring to from the Ma website. Don't ever believe what you hear from a LEO, gunshop, or website WRT mass laws without verifying yourself first. I posted this link on another thread here. check it out. Most of the nuances of Mass laws are talked about somewhere in here.
Massachusetts Laws