"Fasting for religion is also another poor reason to fast..."
Lemme guess - you must not be religious, right? Well, don't knock it till you try it!
Regarding losing 60-70 lbs in 3 weeks, you might be out of luck, but then again... the heavier you are, the faster the weight peels off, so you might have a chance (albeit thin).
Some other suggestions to win the bet:
donate blood!
donate a kidney!
shave your entire body (gotta be worth a couple ounces...)
If all else fails, do you really need both of your legs? (just joking!)