My wife lost her medical due to being diagnosed with diabetes she was 21 years old and weighed less than 130lb.
Contrary to popular belief, you CAN get a medical as long as you aren't insulin dependent. My wife was initially diagnosed as a "Type one and a half" and controlled her sugar with a radical diet (cottage cheese?!?) and exercise, in addition to taking Metformin for a couple months. She actually got a First Class reissued to her, but two months later she could no longer control the sugar well and decided to go on insulin because her career wasn't worth her life, especially at that age.
If its not Type I, the best thing your friend can do is what was already said - lose weight yesterday, drink nothing but water, do cardiovascular exercise at least 30 minutes a day, EVERY DAY...45 or 60 minutes would be better. Cut waaaaaaaaay back on carbs. Drink very little or no beer, only dry wines if any alcohol.
I highly recommend Virtual Flight Surgeons; for $40 they'll answer every question your friend has and step him through the process, even assisting with the FAA. VFS is the same people as ALPA Aeromedical, and while AOPA does great work (I've used them as well for various stuff) $40 is a low price to pay to have an MD walk you through this process.