Re: My friend is getting engaged too soon... what do I do?
If you believe he genuinely doesn't know what he's doing, or that the girl is destined to rip the heart out of anyone unlucky enough to marry her, or he's acting out of insecurity more so than love, then your concern is understandable. I suppose it also matters whether this is solely his idea or if she's been dropping hints, and if the latter, would he be proposing if not for the knowledge that she's expecting it...
I might take the devil's advocate position about "way too fast," though. They're 25. That's not too young to get married. At the very least, they ought to have formed at least a solid working knowledge of what they want in a marriage partner. And based on your timetable, they've had three months of dating followed by six months of living together. It's not a lot, but it's more boots-on-the-ground courting time than was spent by a lot of people who have had very long, happy marriages. If it's one of those relationships that's been just right since day one, it may well be enough time to feel sure. Of course, life has no guarantees.
Not all who wander are lost