I have been running two of my friends through a campaign. I would say it's average to high magic level. In fact, it's a modified version of the Rise of the Runelords. One of the party members is a rogue and is totally in love with the +1 returning cold iron dagger he got for killing the quasit in the Catacombs of Wrath. Both players have been facilitating the absence of a full party with NPCs they've met along the way. They are level 5.
Another buddy of mine has always wanted to join us but his schedule always conflicted. But coming up next weekend he's free so I'm setting up an epic marathon game session for all of us.
Here's where I need advice: He rolled up a fighter. He's specializing in a greatsword (he wants the chance to cut enemies in half). I had him roll stats exactly how all the other characters rolled stats; 4d6, eject lowest die, roll 8 times, eject two lowest rolls. I kick myself for not using a 15-point buy, but what's done is done. The first two players rolled decent stats, nothing crazy so I dodged a bullet. Now comes my fighter friend. He rolled hot. 16, 16, 15, 15, 14, 13, 11, 9. That's stupid good and I immediately worried that after he got rid of his two lowest rolls he would steamroll past the other players.
So I made a deal with the devil. I told him that if he took the 9 and the 11 instead of the 14 and 13 I would let him have a +1 greatsword with one +1 special ability. Ug, then I started looking over the special abilities and some of them are highly situational, requiring you to have certain abilities, but others are like holy crap, that's powerful! Let's be honest, a +1 returning dagger shouldn't be roughly considered the same power level as a +1 holy greatsword! They both have the same number of enhancements.
Now I'm worried the other players are going to look at the fighter as getting a big hook-up, showing up with a flaming sword.
Knowing all of this, what would a good DM do? Let him keep the stats? Trade them for a sword? Something else?
I have my guys roll stats too, and they usually end up with pretty absurd stats... (something I'm working on curtailing) but honestly my guys roll low enough for it to almost not matter.
I think the +1 Flaming greatsword would be fine. Flavor it as a legacy weapon/ heirloom weapon etc, and I daresay the campaign would work just fine. Especially considering he's a fighter, and if he's a full plate/ two hander, he gets much less out of having high overall stats than, say, a monk would.
Think of it this way:
Wealth by level for a 5th level PC is suggested at 10,500 gp. The sword cost is ~8,350 gp. So no worries there.
The +d6 fire damage is an average of +3.5 damage, per hit. Pretty good bonus, BUT he's not always going to hit. And fire is the most-resisted energy in Pathfinder (long haul) so the fire damage will eventually barely matter, especially as they level even higher.
Don't sweat it. If you think your other players might be jealous, try to find similar item(s) in the near future for them.
An extra d6 is about 3.5 damage, versus the most common immunity in the game. It delays his weapon upgrade. Aaaaand as a fighter, the only thing he can really contribute to the party (unless he reaaaaally knows what he's doing) IS damage. And at level 5, frankly, 3.5 damage isn't that big of a deal.
One of the party members is a rogue and is totally in love with the +1 returning cold iron dagger he got for killing the quasit in the Catacombs of Wrath.
The rogue likes a weapon useful for sneaking and sniping. In a non-sneaking and non-sniping environment, it is pretty meager damage without sneak attack. Introducing a great sword wielding fighter reduces the chances for sneaking and sniping. Making that great sword flaming negates sneaking by a lot more. The rogue is going to start losing the opportunities to toss the dagger and be forced into melee more often to get sneak attack.
The fighter, with just a plain great sword, 18 STR and Power Attack is going to be hitting for 2d6+15-ish at level 5. Compare that to the rogue's dagger toss for 1d4+1+STR (after round 1).
Those are the comparisons you need to bring up with your players. Giving the fighter a +1 flaming great sword is small potatoes compared to the comparisons above.
I'd let the fighter keep the stats. It will help to round out his character by allowing him to readily keep INT and CHA to boost skills.
You might also allow the rogue the chance to swap to two weapon fighting instead of potential ranged feats. Mention the feat Gang Up as a means to get even more sneak attack.
Not sure what sort of damage your other players are doing, but a fighter is nowhere near the top of the power curve, extra 3.5 damage or not. The fact that it might overshadow the rogue is a testament to how situational a rogue's effectiveness is, not how great the fighter is.
Forcing the fighter to take low rolls in his dump stats isn't going to materially affect the game one way or the other.
The thing to realize is which stats are going to be his lowest.
14 and 13 is 14 int and 13 cha, so he's not cha dumped and he gets 4 skills.
going with 11 and 9 means 2 skills and a penalized cha. Honestly, I'd take the sword instantly, A lot of 20 pt buys for fighters have like 7 and 7 in int and cha, so 11 and 9 is an upgrade over normal still, and getting free 8,000 gp out of the deal is just gravy!
Honestly, those high rolls have diminishing returns as they are boosting things that don't really matter for the fighter. The slightly smarter and charismatic fighter is effectively the same as the dumber uncharismatic fighter, just a teeny tiny bit more useful out of combat.
Considering how rough RotRL gets towards the end for people who want to fight in Melee, I don't think that letting the fighter have good stats is going to be a problem. That problem is probably going to afflict the rogue anyway.
Is the Rogue Unchained? If not, letting the player rebuild as an Unchained Rogue is going to help a lot.
I've run RoTR, with 6 PCs, and allowed a 25-point-buy, and managed a few deaths. So I'm comfortable saying that as others have said, the stats might not be that big a deal.
That said, if you really want to go ahead with toning him down but wanting to stick with his stated theme, I'd suggest inventing a unique sword for him. Basically, I'd propose a +1 greatsword that does 3x damage on a natural 20 threat. I'd probably note it as /19x2-20x3 or something. 5% of the time he'd produce a super-critical threat, which fits the "I want to cut foes in half" theme, but isn't mathematically as good as keen is. The illusion of a great value, without the power. <Grin>
Let's be honest, a +1 returning dagger shouldn't be roughly considered the same power level as a +1 holy greatsword! They both have the same number of enhancements.
That's because they are not. Returning is a +1. Holy is a +2.
Tell the player "good try, now choose an actual +1 special property this time."