You give the advice to water diet to a person you don´t know anything about.
To fast craves a lot of discipline, water fasting even more.
This person wants to loose weight within 30 days. That sounds as a person who does not want to do a long term change of habits - just like the majority of us.
Do you realize he/she might just start to drink water and work out without a doctor´s advice? You don´t know anything about the health problems this person has. Drastic advice as water fasting to an unknown person I don´t think is a very good idea at all.