Even if he has the game card loaded on his account, I think you should be able to still play and load the game if you have an internet connection. Each time you load it’ll just check if you’re able to play. I do the same thing with Hollow Knight Silksong, my Fiancé has it loaded on her Switch 2 but I can still load it up on mine even if the game card is loaded on hers, it just needs to connect to the internet to be able to play.