Disabling password in recovery mode, is default password restored when re-enabling?
You can reset the password via Recovery Mode or Single User Mode, but you would need to know the original password in order to change it back. 

Another important issues comes to mind if they're using FileVault. You will not be able to access their information by changing their password (and you'll likely break other things trying).

Other options would include:


Making a secondary account (if the issue isn't specific to that user account)
Repairing the issue via Target Disk Mode (if the issue is fixable that way, and they aren't using FileVault)
Obtaining the password, or having them change it to a temporary password they can share with you


Note: If they aren't running FileVault, and you end up changing their password, you'll be prompted to wipe out their Keychain on every boot (which means they lose everything, or you continuously see Keychain errors).

The third option, i.e. obtaining a correct password, is best, and standard practice for repair shops.