Personally, I don’t think it’s possible, at least not in a healthy way. And even if it is possible in the short term, the long term health consequences are detrimental. Extreme, quick weight loss isn’t in my scope of practice.
You might not want this client. There will be some hard discussions that need to happen, and when you are real with him or her, you might have to agree to part ways.
Have a discussion regarding what is possible and realistic, given the client’s existing weight, ability to exercise (injuries?), time and willingness to exercise, AND ability and willingness to change eating habits.
Then, have a discussion about short versus long term results. Although the wedding might be the reason this client suddenly decides to lose weight, as others have written, weight loss that isn’t backed with behavioral habit change will come back.