Are you SURE that there is no password?
If the Mac is set for automatic login, and you don't see a login screen at boot (Your wife's Mac goes straight to the desktop, without seeing a login screen), that does NOT mean that there is no password, only that a password is not needed at boot.
Try this: Open System preferences, then the Users & Groups pref pane. Click the padlock in the lower left corner.
Does that let you unlock by just pressing enter (and no password entered?)
If that allows you to unlock, then, yes, you have no password. Some operations NEED an admin password, so here's a workaround to do that (and what I would do)
DON'T add a password for your wife's account, if you think that would cause problems. If you add an admin password to your wife's account, you can't go back to "no password" on that account. What CAN you do? Add a new account, in that same Users & Groups pref pane. Make it an admin account, with a nice password. Log out of your wife's account, then log in to your new account.
Make your bootable USB for High Sierra. Log out, then **restart**. If you have automatic login, the Mac will boot back up to your wife's account. Your added account takes up very little space, and you can leave it there, in case you need a fully functional account again.
