No, as a dealer you may sell a long gun to an out of state person, as long as that person is eligible to buy it back home.
But someone from California cannot walk in and drop a California license and buy a rifle they would be prohibited from owning in California.
That’s a well known rule, and the store will take it in the shorts on that.
The Government’s failure was not having his violent felony records from the USAF (where he strangled his wife) available for instant check. Also the USAF did not push the proper legal remedies when he showed up at Cannon AFB to kill his senior officer and then some instances of extreme animal cruelty were not documented and pursued.
The bottom line, he should have been in prison anyway.
From what I see, the rifle he purchased is legal in Colorado. High capacity magazines are what is restricted, and you can go to any sporting goods store here and buy the magazines without any check, they are not a firearm. The suit sounds frivolous to me...