Sure, it can and has worked for generations. It's just not really the norm anymore, with people getting married later. Do you think they are ideally suited to each other, or do you think they should be open to other relationship experiences beyond high school? It's not really your business to say anything, I don't think, unless you really don't think they should be together for their own good. Of course you can always mention there's no rush to get married, though your friend should know that anyway...

Sure, there's no real reason why they can't work as well as relationships that are formed in college or in the "real world." I mean, usually the objection is that people just keep their h.s. sweethearts and don't meet any one new or see anything else of the world, and then get dissatisfied later on, do you think that's the case with your friend? Has she dated anyone else or sown any wild oats in the year and a half since you graduated? If so, and then she's come back to this guy, there might be a better chance of their really sticking it out. But I have known plenty of people who've been married to the same h.s. sweethearts for years. I guess it just depends on what kind of person they are, too...

I know of some that have. I know a couple that met in high school and are one of the most happily married loving couples ever. (They even grew up across the street from each other and now live in his dad's house.)..
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They're a really great family. I think it can work. But I also think if they're meant to be together they can wait a little longer. The couple I know waited until the husband graduated from law school...