@ DarthAve: That is pretty rough. Rather unsettling to find after a period of time.
If experience has shown me anything, we all change a lot from 18 to 25 years old. So until both partners of a relationship of a couple are at least about that old, I would not count on someone else to hold steady. Especially if it were a question of whether or not that person will become a snake in the grass and do something like this.
Not saying don't live (by all means, *do* live), just in terms of expectations and human behavior this is usually what happens.
In a year you'll look back on this and see him for what he really is. And laugh when he comes trying to crawl back to you b/c she found someone 'better' than him. Wish you well.