What is the most weight you have ever lost?

Oh and please answer this question: Is it possible and healthy to lose 3 pounds consistently for 9 weeks? I just wondered if that was healthy for my friend? She wants to lose 27 pounds before her vacation. What do you think? Can it be done? Answer this and best answer will be chosen. P.S. She is 173 pounds and is 5 foot 7 inches.