Simply put your friend will need to develop an excellent technique for their swimming, cycling an running combined with exemplary levels of endurance.
In terms of pushing oneself many consider an Ironman as one of the hardest tests of endurance in terms of it's physical nature which your friend can train for over time. The main hardship is likely to be mentally to keep going during their race as well as adhering to a very time intensive training schedule
Good luck to them
You'll have to write a hub on how they get on