A) it is not illegal for a US resident to give a Canadian resident a gun. There is a procedure for it, which afaics was not followed.
B) That's federal law, not state. ATF, which is a federal agency, is obviously aware and has declined to prosecute him. Ask them for answers, not r/Maine.
C) we're not talking about a black market arms dealer here. We're talking about a guy who trusted someone enough to invite them into their home, work on their home, and gift them a firearm. This is not the crime of the century, it's a procedural oversight.