Well, burning fat is I guess about any normal weightloss regime. It's about eating sensibly and exercising regularly.
Your friend should eat healthily. Consuming 5 portions of fruit and Veg a day. Consume plenty of water. Maybe if she has big portion sizes, she should think about cutting down.
Using the Wii Fit I doubt will have a drastic affect on weight loss (although I have one myself and love mine!) although use plenty of the aerobic exercises. Especially the jogging on the spot one.
The wii fit also tells you that the best way of keeping fit and healthy is to do other excerises outside of the game. So maybe joining a gym, where some instructors will explain the best ways. Or maybe just going out jogging.
She will not see big improvements straight away. (Rome wasn't built in a day) however, if she wants to have a healthy life style. She should keep exercising and eating healthily.