He has Android 5.1.1 with the Find7Spectrum_12_160601_Beta build. Everything pretty stock, so no root I think (I don't have the phone with me right now). The partition type is the old one. Actually, one of the reasons he want to flash a custom rom is "to get more space to install the apps" 
Where do I have to start from? Thanks a lot in advance!

Where do I have to start from? Thanks a lot in advance!