No, the new version of Fiddler is not admin-only. Fiddler's installer is requesting administrator privileges for ages. However, the new version of Fiddler is installed on per-user basis.
In your case what happened is when you choose to upgrade, you should've been asked for admin credentials. When you elevate the process by giving the admin credentials the installer knows that the administrator account is installing Fiddler and it installs it to administrator's AppData folder. This is a known issue and it's already on our backlog.
What you can do at this moment is to log into the administrator account and uninstall Fiddler. Then download the installer from here
and run it. When asked for admin credentials go ahead and type them. Then you will be asked where to install Fiddler to. You can change it to wherever you want (it is still highly recommended to install it to AppData, so changing only the username in the path should work). Then you can continue using it as normal through the user account.
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