Without more info, I'm not sure if he's totally afraid, has always had issues with holding poo, or if he has a little potty of his own, so sorry if some of my ideas aren't applicable.
First, many kids are afraid of full-sized toilets (our daughter was and still prefers her potty most of the time). If that is the case, I would recommend a small potty (we LOVE our baby bjorn ones).
Second, has she tried an incentive for him? Early on we had a potty chart and also dum-dums for a poop. Later, when pee training was done, we had a poop chart that our daughter could add stickers to. Our friend gave a hot wheels car for the first poop ever on the potty and now just gives lollypops for each poop. Another way is to combine those ideas, a sticker per poop, once 5-10 stickers, then a prize. Etc....
Third, it's very unhealthy for him to hold it that long so I would do anything to make him have to poo more. Our little girl has gotten constipated on occasion since she was a baby, and a little prune juice works wonders. :-) It doesn't take much. We keep the 6-packs of the small (less than 6 oz.) cans of Sunsweet prune juice on hand. That way, when we need it, we give our daughter half a can and store the rest for the next day. Usually within 15-30 minutes, she has a big poop. Don't give more then that, however; because he could get the runs.
Finally, tell your friend to make sure she relaxes about it. He'll figure it all out sooner if she doesn't make a big deal out of it. There are very few things toddlers have control over: sleeping, eating and pooping; so he might just feel like he isn't allowed to progress at his own pace. She doesn't want to make it a negative experience - instead focus positively on the goal.
Hope some or all of this helps. It will happen, I promise. :-D Oh, and when they're ready, it usually all happens so quickly that you can't believe it was as easy as it was to potty train them.