I bought a digital game (Rainbow Six) today and I must download the data from play station network. My friend owns that game on a disk. Could I install his game on my ps4 istead of downloading the data from psn?
No it won't work. You have two versions of the game: digital and physical. Your friend has physical and doesn't need to authenticate through Sony's servers; digital versions do.
It won't work. I own Bloodborne digitally and physically, but when I want to use the digital version, I need to delete the whole game just to redownload it.