For your boyfriend here is a site about fasting and Master Cleanse with quotes from 11 MDs. A lot of the info is from MDs. Has a link to a book by MD who has fasted patients for 3 decades.
The book tells about how there has been a lot of recent research on fasting by MDs. One of the 11 MDs has his own website where he says that fasting [not medicine] saved his life. Site also tells about things that can go wrong and chances of it. This site is not the opinion of one man but the experiences of many including a guy who fasted 40,000 people.