I think therein lies the problem.
None of us know if the player in question made his decision based on knowledge of the system or ignorance. If the player conciously made the choice to play with lower stats, that's perfectly groovy. However, if the player did it because he thought that having all stats at 12-14 was mechanically better than having a better prime stat, then a bit of advice is not remiss. In other words, until you know why he did what he did, you cannot really know if it was a mistake or not.
Which gets back to my original point. Ask. Nicely, politely, in a whisper during the game, just ask. "Why did you put a 14 in your Int?"
Would people honestly get offended by this question?
I have no problem with people asking me why I designed a PC the way I did, as long as the question isn't pointed.
e.g. "Why did you put a 14 in your PC's Int when that means he's going to suck?"
If the followup to my explanation is polite, no harm, no foul.
If, OTOH, I get the stinkeye and unsolicited advice, then I'll be telling you where to go.
It's not his choice at all. It's a group game. Being the lone-wolf chaotic neutral my way or the highway "I'M TEH ROLE PLAYA!!!!!11!" type is disruptive and annoying.
Which as was shown in another thread is fine if that's what the group dynamic is all about, but most people think such people are just selfish dicks.
Its a group game, but people bring their own pieces, as it were. And unless the
DM says otherwise, what they bring to the table is none of your business.
For all you know, that seemingly suboptimal PC has a HR boon from the DM that will more than offset the hinderance.
But even if it doesn't...its
still none of your business.
If you think it still is, consider what you'd do if you were told by your DM that you were using point buy for stats, but your main class' key stat had to be the class minimum. Or what it would be like to be the sole powergamer in a group of amateur thespians.
If you don't like someone's playstyle so much because it doesn't mesh with your preferred style- because he's the lone role-player, for instance- then find a group in which everyone plays like you and enjoy.
But don't piss on someone else's enjoyment of the game.