Just like with most things in life, timing greatly impacts the outcome. You can't get your dream job unless its 1) available and you're 2) able to take it because various factors worked in your favor. Timing.
Relationships are the same way. Perhaps you meet somebody great but you're in a relationship. Well if at some point your relationship falters and they are available, then voila. Not that I'm suggesting anybody go out there and try to meet new folks while in a relationship, but it happens. But due to circumstances - timing being one of them - things can't be pursued.
Same with your friend who found "the one", he recognized it but at the time and point he was in life, he couldn't do anything about it, or realized that it wasn't the right time to pursue. So he waited and when the time was right, he took advantage and was lucky enough that his girl was also available. I do think that if something is meant to be, it will be, but there are things that also have to be in place to make sure it can be. And the right timing is one of them.