It's one thing for a college student to carry on a long-distance, online only relationship. A first round draft pick with a 7 figure salary isn't going to be able to keep that story going.
He could say they broke up, but people will eventually wonder why he's never seen with any women. If she "dies", then people will chalk up his bachelorhood to mourning.
Do not forget the fact that he is Mormon, as I think it must be considered when assessing and evaluating how Manti would have dealt both with women in general and, particularly, with Catholic women on ND's campus.
He might have actually been committed to saving his virginity until marriage. He might have actually had real feelings for this mirage. The person carrying out the hoax obviously had strong reasons to insist that Manti not visit, which might be where the car accident and cancer angles get introduced (these two things might lesson Manti's suspicion of unfaithfulness, while also increasing his deferring to her desires and insistence on not visiting).
It is highly convoluted, there is no questioning that. But, to say that Te'o somehow thought he would have needed this story, and knew he would need it 2-3 years in advance, is a stretch.