[Q] Failed chmod? topic

I recently tried to root a friend's phone (a MT6572), using the Root Master Cekas app (it won't let me post outside links, as I recently made this account, but anyone can find a tutorial by searching "root master cekas tutorial" on Google, to see the app used) . It succeeded, and since it installs the Cekas app as a SuperSU, I was going to replace it in /system/app with the Chainfire SuperSU APK file (which I have done before, on an old tablet, and that worked). In a Terminal window, I used the command "chmod 644 /system/app", thinking that was the correct command (last I did it was around a month ago), but right when I did it, all apps started to close. He couldn't do anything, so he tried restarting it, and it freezes whenever he tries to do anything.

So my main question is, is there any way to use ADB/fastboot without having USB debugging active (he can't get into the settings menu) ? Or is there any possible way to turn it on remotely from a PC?


Thank you for your time,
Takumi