All you did was follow the instructions of the scammers at the cost of your funds. The used Atomic wallet to deceive you, I have seen similar scams attempts using trust wallet but I never fell for them because I knew from miles away that they were scams.
Whether he was scammed in the name of Atomic Wallet support or somewhere else, we can't be sure, because based on what he has said so far, he contacted the official support and also downloaded the official version of the app, and the support team never asked for his seed phrase but asked him to import his wallet in the Cosmostation. I don't see how they can transfer his funds if they didn't get the seed phrase themselves.
So, I believe he most probably got phished or something before this issue or while he was in contact with the support team of Atomic Wallet because there can only be two things, either someone had access to his wallet through the seed phrase or the Cosmo wallet team accessed his wallet and transferred the funds.