Quick weight loss – within a week of training – is usually the result of glycogen/water accumulation. Its normal. That said, I think the thing all trainers need to remember is that there are 168 hours in a week. You see your client, what, about 3 hours per week? Even if your client works out 7 days per week, its hard to out exercise eating too many calories.
There are only 3 things that burn calories in any given 24 hr period
1. resting metabolism
3. thermic effect of food
of those, we burn the most calories via our RMR. Exercise only accounts for about 30% of total daily calories – and thats in people who are used to exercise.
These facts provide evidence that most people can not exercise there way out of eating too many calories
I suggest that you recommend that your client use MyFitnessPal or a similar app/website to track calories. If he/she has a fit bit, that software is also pretty good.