Your friend used "watts per second" where he should have used "watts per hour". As a result his numbers are far, far off.
Watts per hour also does not make sense. As others have noted watts are a measure of energy per time already, so "watts per " would be some funky kind of energy acceleration measure that nobody uses 
.It really is as simple as:
100 W = 0.1 kW
If we use 0.1 kW for 5 hours we would get:
0.1 kW * 5 h = 0.5 kWh
Don't feel bad though, I've even seen electricians getting confused by this
