it seems that you are only just starting with this client, so you probably won’t know her all that well yet. You are aware, though, that emotions and her weight have a relation.
If she is okay to weigh once a week, that’s great. Susan’s point of only shooting for 20 lbs. at a time is right on. Nobody can visualize losing 150 lbs.
While the points about body fat are valid, at that amount of weight, any estimate of body fat will be highly inaccurate, and it will be better to go with weight alone. I also would only use one measurement; the waist would be best.
Be prepared for a very rocky road. At that weight, the client is likely to have health and/or mobility issues that will make the start difficult.
I wish you and your new client best of luck.