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apart from the other correct answers (you can’t „protect“ a seed phrase once you gave it to someone), i would like to point something out:
> Disclaimer there is zero funds in the wallet and probably won’t ever be.
So in other words, your friend has a seed phrase _to an empty wallet_.
Turns out, there are billions upon billions of those things. There is nothing that binds you to this seed phrase in particular. You can throw it away and then it’s not your seed phrase anymore, it’s your friend’s (if they choose to hold on to it).
Put differently, the situation is the same as if you and your friend made some sort of pact to use this particular seed phrase together. Because you never used it, and never put funds into it, there is no reason for you to uphold this pact. You can just break away and put your future money somewhere else.