Wow, unreal. I dont that guy was (is) a troll...it sounded pretty sincere to me.
Of course when sosuavers read a story like this it's a knee-jerk reaction to throw the baby out with the bath water and say "I'm NEVER getting married, pat test, pat test, pat test!" There is certainly nothing wrong with either of those things, but anyone with half a rational brain knows that marriage is FAR from a sure thing, even under the BEST of circumstances.
What I think was probably a contributing factor is the age at which they got together and length of their relationship. It shouldn't be too earth-shattering when one person in a long-term relationship that started in high school gets an itch to go out and try something "adventurous"...aka cheat. NOt necessarily a rule, but not a rarity either. She lied to him for the same reason anyone lies---she was too much of a coward for the truth.
Tragic story...hope they at least pull through for the boy through his health problems.
-As an interesting side note, I remember our genetics lecturer telling us some INSANE stories about people who would come in for genetic testing of some sort and all kinds of crazy cheating and relationship dirt would come up. She said one couple came in to have their child tested for what turned out to be a very rare disease that is mostly seen in consanguinity (inbreeding). Well, turns out, this couple were father and daughter. Apparently unbeknownst to both of them...you couldn't make this sh!t up. She also said the paternity thing was very common.