The SATA and Molex connectors on the power supply are for the hard drive(s) and/or DVD drive(s). For example, if he had two hard drives and one DVD drive and they were all SATA, he'd require 3 SATA connectors. The power supply you picked out had 2 SATA and 4 Molex. Though there are adapters to change SATA to Molex and vice versa, I don't see that the power supply you picked out included any.
***Here*** is a picture of the adapter. Considering the age of the E521, it might have all SATA connectors. The hard drives are certainly SATA according to the manual but I'm not sure about the DVD drive.
If he got a more powerful card than the HD 5450, he might need a separate connector for that. But none of the HD 5450's I looked at required one.