wait, what?!?!?!?

so i'm confused. it could be b/c i only saw like the last 20 minutes of the movie. i turned the tv on and it was at the part where they are driving out to the little shack by the beach so she can go talk to the guy (who isn't there b/c it's brad frigging pitt. it's sad that after watching only 5 minutes of it i knew that pitt was the other guy). i've gathered from reading the other messages here that he has a rare skin disease and is like allergic to sunlight or something. but my question is, how the hell is he out in the sun then and nothing happens to him? and why does he have to leave at the end? and what the hell is wrong w/ his mother? lol, maybe someone should just tell me the whole story since i know the ending.