Dear Aneesh,
I think what Urvashi, Asha a few others have mentioned makes a lot of sense.
I remember one line of a dialogue from an old movie [I think Sangam]: if you love someone, set him free [those days it was the male who was supposed to be the wanderer!!!], if he comes back-he is all yours; and if he doesn't, he never was.
In the case of your friend[shall we call him as 'A'?], nature [or if you wish God] did that for him. I agree with Asha's point about: what if they got married and then she strays? Let me tell you on this: Asha just mentioned it as an argument, but I have seen it happening FOR REAL-only that the reasons were different.
And I can empathise with A, since the male in question in the incident I am referring to just lost interest in life, etc,etc.....
There's one quote that I know is DEFINITELY true [at least in my life it was..]:
When one door closes another door opens; but we so often look so long at the closed door, that we do not see the ones which open for us. --Alexander Graham Bell
[I hope you know who he was].
Looking @ it from the spiritual angle, it gives the essence of the Gita and if viewed from purely a practical angle that's reality in life.
Going by what you mentioned about the incident, this is the lady's nature/psychology [I think I am at least 90% right in this]:
1) She is of the type who can't/doesn't want to take responsibilities in general. She only wants to be the 'taker' rather than the 'giver'.
Reasons: My feeling is the topic of the parents MUST have definitely been discussed between her & A earlier and she must have made up her mind quite earlier. You can check up with A on this. Love and/or marriage isn't just running around trees, going to movies, etc. It comes with the associated responsibilities, just as a job would or for that matter any other relationship/situation would. My responsibilities would be something, and your's could be something else and so on and so forth.
2) She is of the type who doesn't/can't responsibility of her actions--whether good or bad. [Different from the above]. And she is very self-centered/selfish who wants 'her' interests served, AT THE COST OF OTHERS.
Reasons: She too was a part of the 5 yr affair. So when she wants to break it, she didn't have the guts/courage to tell him that.....and most likely she was just using his parents situation to justify/avoid the delay in 'her' marriage.
We see this behaviour pattern mostly in politics--the BJP takes off on Congress, and then Congress says--look you did that..& that..& that 6/7/8 yrs ago. All focus on the present is left to the air.
I could go on, but just tell A to get his focus back on the present.
He needs to look around [and you as his close friend MUST help him do it......another quote here: A true friend is the one who walks in, when the world goes out] to see which doors opened for him. Maybe there could be some opportunity that he ignored or left while he was with that gr8 lady--to satisfy her. Let him think and he could recollect, and he can renew his efforts to take up those opportunities. Just tell him that one shouldn't allow others actions or our own past to DICTATE our future--which would depend on what we do TODAY; just as our today depended on what we did in the past [it's both the karma theory as well as the reality].
If there's anything I could do out of this portal too, let me know.
Rgds,
TS