How fast are you supposed to lose weight on a weightloss plan?

A friend of mine signed up for some ridiculously expensive weight loss program; one where they send him the food he’s supposed to eat and supposedly boost his metabolism.

Originally I was worried he was scammed and wasn’t going to lose much weight if at all. Now I’m worried he’s losing weight too quickly. It’s been less than a week, and he states he’s gone from 300 to 289, and estimates that with the exception of variable weight (things like water) he’s lost 5.4 pounds total.

I’m no dietitian, but that seems like a lot to lose in just a few days. He’s not very active due to a knee injury, so most of his weightloss is due to whatever they’re sending him.

Is it normal to lose around 10 pounds in a week on a weightloss program, or am I worried over nothing?