Keyboard layouts on Mac Mini

I'm starting this thread in order to help my friend who is new to Apple and just got his first Macintosh: a Mac Mini.

The thing is: he accidentally deleted all the keyboard layouts from his computer's system folders. He is not using an Apple keyboard nor an Apple mouse. In order to reinstall the layouts, the OS asks him to provide his admin password - which he can't type because his keyboard is not working and if he restarts the computer he won't be able to even boot it anymore since it's gonna ask him his password.

Does anyone know any solution other than reinstalling the whole OS again? And if this is the only solution, how to do so if the Mac Mini doesn't have a CD or DVD drive (I never used a Mac Mini before since I have an iMac myself).

Thank you guys!