If his account is the only one on the Mac, then it theoretically should be an admin account. Open the User & Groups System Preferences to confirm. If it is an admin user, then you can create a new macOS user account for yourself.
If there are no admin user accounts, then there are ways to create a new one, but it may be better off to perform a clean install of macOS instead since it may be a sign of some sort of issue with macOS.
Besides a 2015 model can run at least macOS 11.x Big Sur or possibly even macOS 12.x Monterey if you have the Late-2015 model. Definitely would be a good idea to upgrade macOS to the latest supported for this hardware since you will be having difficulties accessing websites before too long because third party browsers will drop support for High Sierra at some point.