Once you tie a key to a Steam account it cannot be un-tied, and cannot be tied to a different account.
Uhhm I'm talking about using a key after linking it. Like I did;
I linked my key to Steam and used it once within Steam, then never used it on there again just used the old launcher with the same key.
Not trying to input it on Steam but on the launcher version
So again, it should be fine if he has his key written down somewhere; just use the launcher and it should be alright
