Id apply the SMART principle. And inform them that they will not in any healthy way lose this much weight in this allotted time. It is simply not an “Attainable” goal.
But you should temper that will urgency to start today to lose as much body fat (not just weight) by the date desired and for them to feel good about the results in such a short time.
The reality is that most of us wont take action until there are consequences. This pickle they find themselves in is of their own doing. They simply waited to long. Having ownership of their personhood and acceptance of the consequences of inaction for so long, leaving such a short window for change, is in fact their doing.
Be firm with that but use it as a “teachable” moment. They will grow if they listen