I have a friend of mine who is looking to build his first rig. He is looking to use the MSI Enthusiastic Gaming Intel Z270 M7
http://amzn.to/2Dj4tWU to build a 6 card rig.
Cool Board with LED arrows
Can he plug risers into the full slots?
Yes, this is possible, just be careful not to plug the risers in reverse, you might end up frying the board or worst your GPU.
If so is there something in the BIOS he needs to change to work properly?
Basically the 4g decoding and the Bios set to gen 1 are the basics, however try not to touch them at first and install your GPU's they might work flawlessly without any configs, less headache.