Three things that I would guess :
1: BIOS Settings
Remove the WD Drive from the boot list and set this port to be something ELSE than AHCI.
Spike, the jumper thing is a good guess but SATA drives don't have them, only IDE ones. SATA pins are for enterprise features, like spread spectrum clocking.
2: He plugged the WD drive into a SATA 3 ports (that is just a guess, knowing that it should be retrocompatible). Usually the first ports have a different color than the last ones.
I would suggest him to plug the SSD into the SATA 0 port and the HDD in one of the last set of SATA plugs. Otherwise he could look in motherboard manual for specific use of SATA ports.
3: As ridiculous as it looks, a faulty SATA cable. I have seen my share of those. Stay away from those rounded ones, as I had more problems than flat ones.