He could legally answer no to that question. He is not an unlawful user nor is he addicted to the drug, (anymore than any user of pharmaceuticals is addicted). State law should always trump federal law, unless it conflicts with an individuals rights, that's what the founders wanted.The transfer of firearms through the NICS background check is controlled by the Federal Government. Regardless of the varying state laws the Federal Government has decided that marijuana is illegal, for medical or recreational use. Question 11e on the 4473 asks if the purchaser of the firearm "is an unlawful user of, or addicted to, marijuana or any depressant, stimulant, narcotic drug, or any other controlled substance". In the eyes of the Federal Government there is no such thing as a lawful user of marijuana since it's possession is a crime.
Not saying I agree with it, just stating how it is likely being considered. It wouldn't surprise me to hear that the BATFE pays this guy a visit for "lying" on his 4473 (if he answered no to question 11e).