An interesting point of contention!
At some tables, the mechanics come first and the flavor covers them. At other tables, the world is primary and the mechanics are mean to support it. The first mindset makes reflavoring much easier. The second makes reflavoring feel fake.
Personally, I feel both perspectives are valid ways of approaching the game. But a difference in view regarding the primacy of world and mechanics can cause contention when present as a single table.
Well I do in general think flavour is free and like reflavouring, however, for things which exist I find it a bit harder to justify because I want things which are different (like different weapons) to feel different.
Its more a "ok thats the only thing we can do in 5e so we can suggest it." I dont think its a particularily good solution for this situation, but its the only one 5e allows.
Because telling a player "oh yes put your points in dex and str" when I know that this is just an incredible bad advice for me is not a solution, because it will just frustrate the player later.
I'm confused by the player's question, because this is already handled by the system? Just put their top stats in Strength and Dexterity. They'll benefit from it via the various mechanics of the game.
What mechanics? 5e is made that strength and dexterity have anti synergies and putting points in both will feel wasted.
Strength is literally the worst stat in 5e. Its a rare save (and one of its use, getting not grabbed, overlaps with dex because it can be done with either dex or strength), it only has 1 skill associated with it (athletics) which is not that often used and when for things like swimming and climbing ehich can be overcome with other ways (spells, swim/climb speed, climb gear).
The only 2 benefits of strength are being able to use weapons including 2 handed weapons and wear heavy armor (with no speed penalty).
If you use a 2 handed weapon and heavy armor, strength is required but then the value of dex goes down A LOT. You no longer gain armor from dex, you no longer need dex for attacking or getting out of grabbed, and you cant really use stealth the best dex skill anymore.
Also as you can only really increase 1 stat efficiently at a time dex will lack behind after level 4. (So you rather have someone with not lacking behind dex try to steal or open locked things).
You will be a better "warrior" if you keep your dex at 10 and put points into con (and wis) as secondary (and tertiary) stats.
And if you dont attack with a 2 handed weapon its way way better ro have strength at 8 and put points into con (and wis).
Sure in theory dex and strength give flexibility in weapon choices, but with fighting styles and weapon masteries (and feats) you do want to specialize anyway.
Thw only kind of "Warrior" which does profit from having dex and strength at the same time is a barbarian (who is not using armor) but even then dex is a tertiary stat since con is more important than dex.
And the bonus you get from dex is just passive armor (and initiative) not something you really feel too much in the combat, but I guess mechanically that would be the best case for strength + dex.
(Still compared to other games like DnD 4e this feels disappointing since there is no way to get any bonus to attacks for having strength and dex).
If he has a Dex 18 and a Str 18 he already has a meaningful advantage over a charachter who only has one of those stats high. He is more versatile and will have stronger saves.
I think what you are asking is if my str gives me a +4 to hit and my Dex gives me a +4 to hit should I get a +8 to hit.
I think this problem is a sibling of the a fighter with a 18 str and a 3 dex has the same chance to hit as a fighter with a 18 str and a 10 dex.
Not only same chance to hit but also same damage, same armor and (if stats are put into con and wis instead of dex) higher survivability.
The answer is switch to a game that is more crunchy, or just accept that most games have to have abstract rules.
You dont need a more crunchy game, 5e would have just needed to be more consistently designed.
There are fighter subclasses which profit from con, int, cha, but not all subclasses profit from a secondary stat and none profits from dex (in addition to strength)
The same with rogue and barbarian. No dex barbarian subclass or strength rogue subclass.