Many suppliers offer kits, with parts lists - e.g.
(from <
* Motorwheel
* Battery
* Controller / Battery Cradle
* Battery Charger
* LCD Handlebar Display with control panel
* Thumb Throttle
* RPAS (Removeable pedal assist sensor)
* E-Brake Sensors
* 4 to 1 Wiring Harness
* Motor Extension Cable
* Plastic Wiring Loom/Sleeve
* Cable Ties (10)
* "C" Washers (2)
What you need, and what they supply might be different, and the exact parts will depend on the motor and need to be compatible.
Frankly, unless you know exactly what you are doing, or are prepared to to an awful lot of homework, you will likely save time and money buying a kit and not using the front wheel he already bought. You may be able to find a kit supplier who sells can supply the parts you need to drive the wheel he has.
You probably want to check any local regulations - e.g. <