This is actually a common issue with women who have yo-yo dieted in the past. The scale can fluctuate greatly, so in a way it is refreshing to see people opt out of using the scale as the holy-grail.
I think pictures are a great alternative, as is having the client try on one pair of jeans weekly and document how they fit.
It is also soo much more important that the client FEELS a difference in his/her body. While physical change is a major, undeniable component, having the client understand that exercise and lifestyle changes actually FEEL better (after the first couple of weeks of pain) is huge…that’s when clients really GET it, and want to stay with this as a lifestyle.