Unless that person is open about their weight, this is a really, really hard subject to bring up from a friend stand point. You could come across as incredibly mean and they could attempt to cut off ties as a result.
The solution though is that the person has to have an environment where they feel comfortable working out. So they can make the changes they need to make so that they can feel comfortable with themselves in public. At which point, this type of person usually gains a lot of what might have appeared lacking but they were just repressing.
However, it can be difficult to create a comfortable environment for that person to get in shape. He may not want to be seen running outside, that leaves treadmills, treadmills aren't exactly cheap. Eating can change with willpower but a lot of people don't see changes fast enough with eating, need the exercise. He can start weight lifting with dumbbells but most people, for whatever reason, see this as difficult. They don't know what exercises to do or don't know they can get a total body workout that way so they just don't try. If you can get him working with dumbbells on his own he may improve significantly. The cardio still becomes a problem.