Lots to say here-I'll try to be coherent....
If your friend's goal is merely to finish a marathon, he can realistically do so in less than a year. Consider: if he's not morbidly obese, and reasonably tall, he can practically walk the thing at an average of 5 miles per hour, and finish in a little over 5 hours-jsut as well as Sean "P Diddy" Combs and better than Oprah finshed in the New York Marathon-no big deal at all, at least, to me.....If his goal is to truly test himself, though, and improve himself along the way, he's got his work cut out for him, and I have a few suggestions.
While everyone offered really good advice, one of the first and foremost things I would recommend would be to obtain a gait analysis and coaching in the right way for him to run. Not everyone runs the same, and no, as a certified personal trainer I can say that most personal trainers do not offer this sort of coaching-it makes a world of difference.
If your friend is overweight, he should lose the weight with moderate cardio and weight lifting along with some running/training before undertaking any distances in excess of three miles. Having run marathons at a svelete -and much younger-185lbs., and a muscular but much heavier 265 lbs., trust me,runnning at distance is easier at lower weight for a variety of reasons, but especially on the knees, feet and ankles-while I'll never see 185 or 265 again, I can say that I noticed the difference between 230 and 215-it means that much- That running should probably be on a track, and alternating running 220 yds. with walking 220 yds..
This is also very important-choose the race carefully! My first marathon, more than 20 yrs ago, was the Long Island Marathon-mostly flat with easy hills and good weather, and I finished handily after training for a short time, but it was an easy race. Two or three months later, I ran the Yonkers Marathon-ridiculous hills, hot and humid, and it almost made me quit. If it had been my first, I'd have never done another......this lesson has stayed with me: my first 100 mile race was the Javelina Jundred in Phoenix, Arizona-relatively flat, predictable weather, not too tough. While I've done the course at Leadville,CO., I haven't actually run the race yet, and can expect not to finish-it's got ridiculous terrain, huge altitude changes, unpredictable weather-men go mad!-and it's a wonder no one has died running it yet-but someday I'll finish, and I'm no runnner......at least, not much of one.....
You and he should also look at Carmichael Training Systems-they specialize in taking the non-runner to marathon, ultra-martahon, triathlon and adventure race capable.
Good luck, and make sure your friend has fun!! That is the most important thing.......