Well, Mike's trying to prevent the OP's friend from committing the felony, anyhow. The onus is on the seller federally.
OP: "Private sales" may be fine, but the private TRANSFER is only permitted federally between residents of the same state (again, not getting into what is a licensee). EVERY other "private sale" transfer has to go through an FFL FOR THE TRANSFER - I.E., BEFORE THE BUYER TAKES POSSESSION. With long guns, that can be an FFL in any state (with handguns it has to be one in the buyer's state). When the Fed says you can't do an interstate transfer between residents of different states, that means you taking possession and bringing it back to an MA FFL for paperwork/check is a no-no. It's fine if an FFL does the paperwork and background check before you take possession. Thus your friend can ship the Marlin to a MA FFL, or you two can meet up at an NH FFL. If your friend is brave, or has a non-res MA LTC, your friend can come with you into MA and you can xfer right at that FFL LEO you mention, but given the FFL's other statements, that FFL probably doesn't know MGL Chapter 140 Section 129C (p), so if your friend doesn't have a non-res MA LTC, the LEO may be inspired to arrest your friend for possession of a firearm without a MA license or FID. Everyone in MA forgets about or misreads that section. ANYHOW...
There is your response again. Take it to an NH dealer with your friend and file an FA-10 when you get home. If you take possession from your friend in NH and then bring it to MA to a dealer on your own, your friend's done a private transfer to a non-resident and broken federal law.
THEN, there is MA law, which says you have to file an FA-10 (if you did not transfer through a MA dealer) within X days of bringing it home.