there are a couple reasons your friend lost so much weight
1. fruit is very alkaline in the body
2. fruit is full of vitamins, minerals, and antioxidants (has to be organic)
3. fruit is full of fiber
4. fruit is very filling because it has so much water content, and you won't be able to eat a lot of it and get a lot of calories or carbs
so yeah if you just ate fruit for the next month you would very likely lose a lot of weight if you were fat. absolutely
what i recommend though is a balanced diet. and I recommend implementing fasting strategies to boost insulin sensitivity. you can check out my Ebook in my signature which teaches you how to eat to lose weight.. one of my fasting protocols is a fruit only fast.