My friend purchased a handgun from a dealer,and after the 3 day background check did notcome back negative , they gave him the gun.Almost 4 weeks later he gets a call from anATF agent telling him he should have failedthe background check and has to surrenderthe gun Is this legitimate?

NO. This is obviously a scam. The ATF does not do background checks, the FBI does background checks. Background checks only take a couple of days, not 4 weeks. The government doesn’t do anything by phone. The government would have to take the gun owner to court and win to seize the gun.