When he tried to update to the newest version of Lollipop avaiable, the phone entered in a bootloop.
I tried flashing a factory image manually using this guide, but after it was done, the phone booted and it was still in a bootloop (stuck on the colored circles moving). I tried wiping data/factory reset afterwards and the phone remained in a bootloop state.
I also tried using the nexus root toolkit by WugFresh using the option "Flash Stock/Unroot" with the Soft brick/bootloop box checked, but it also didn't solve the problem.
There were no error messages when flashing, and I waited for 30 ~ 45 minutes after each flashing attempt to make sure that the phone was in fact in a bootloop.
I don't know what to do anymore... Can it be a hardware problem?