Well, I can't say anything about the length of time deal--I met my hubby in september, got engaged at Thanksgiving, and married the following march. 18 years later the rest of the family is still waiting for that baby that was supposed to be the cause of that quickie wedding! lol!
That being said, the best advice I would give you is stay out of it. I know it's hard, you care about your friend and don't want to see him hurt, but he's going to see it as jealousy and interference on your part if you try and intercede. Just be there for him if it does go south.
As for her old high school friends, some people change a great deal from the person they were in high school--some grow up and become better adults, others stay stagnant and never become more than they were in school. I know that myself, I have little to nothing to do with 99% of the people I knew in high school. I'm not the same person I was then. I like the Me I am now much, much better.