I only found it on the German Amazon site, but it looks like this PSU is supposed to come with two PCIe cables, and each of those cables has a 6-pin and a 6+2-pin at the end of it.
Your friend should use the 6+2 from each of those cables for his RTX 2070. If he is missing a cable, I would assume BeQuiet would sell additional ones.
See this link for the photos, particularly the photo of the side of the PSU where the modular cables can attach, and the diagram of wires picture.
https://www.amazon.de/quiet-Pure-Power-L8-CM-630W-Netzteil/dp/B005JRGVBQ