I know what Daryl means - he started up the new Mac, which when it arrived had no user etc., and got it up and running with his wife's user info. So, now, he can't simply set it up using Setup Assistant from the get go, which is by far the easiest, because he has already set the account up. However, not to worry.
There are various options. Probably the easiest is to simply start over - use the recovery partition on the new Mac to wipe it, reinstall the OS and this time when you start it up and start the set up, select transfer information from another computer/backup.
https://www.macworld.co.uk/how-to/mac/how-reset-mac-factory-settings-3494564/ (obviously, you don't need to worry about saving anything from the new Mac - just use the instructions to start from recovery mode, erase the drive and reinstall the OS)
There are ways to re-run Setup Assistant (which only runs once at initial setup) from the Terminal, but I suspect that it's going to be easier to just start over.
The other thing you could do is create another admin account, log into that, delete the account you set up for your wife, and then run Migration Assistant to move everything over - that should also work. You can then delete the spare admin account when done. Presumably you gave the computer a name that will still work. (that won't change).
In theory, Migration Assistant should also work with the "new" account for your wife - asking you whether you want to overwrite things with the stuff you're migrating from the old Mac, but I have had issues with trying to combine a "new" account with the old. Believe me - you're not the first person to make this mistake in your eagerness to get the new machine up and running!!
BTW - the problem with using the backup of the old Mac to clone to the new Mac is that the new Mac likely won't run Sierra. New machines typically don't run with older OS' than they shipped with (unless they're previous generation refurbs perhaps...).