What are you talking about?
Sure you can stat up a Rogue who dumps Int and the skill monkey role... but why on earth would you want to do that?
So you have survivability closer to a Fighter or Barbarian.
The player chose to prioritise Int over Con because they wanted to play someone who was intelligent, but not as tough. Presumably for the same reason that they chose to actually play a Rogue rather than a Fighter with a criminal background. As soon as the DM mentioned that their style was going to disadvantage the Rogue compared to purely combat-focused, resource-based classes, that would be a definite option for someone wanting to shine more in combat compared to out of combat.
As it is, the player went ahead and made the choice to play a less-combat-focused class, and assign abilities in a less combat-focused distribution.
(The answer is probably +10 hp since they're level 9, but I would have to check to know for sure. The main difference is one between the hit die sizes: 10 hp compared to a Fighter or Paladin, and 20 compared to a Barbarian)
But you're missing my point: my complaint is that the Rogue must sacrifice survivability just to gain out of combat capability. (If its in combat capability was any good, this might have made sense. But it's not, so it doesn't.)
Yes. Exactly. The Rogue does sacrifice pure combat capability for pure out of combat capability.
Just like the Fighter sacrifices out of combat capability for combat capability.
Choosing to be best at something involves choosing other things that you won't be best in.
I'd suggest you have a chat with your player. It sounds like they are unhappy about their low performance in combat and don't view their choice to be good at out of combat things to be worth the trade-off. Thus would rather play a more combat-focused character. It shouldn't be too hard to remake the character as a Dex-fighter with superior combat capability.
Instead of talking in sweeping terms about how horrible it would be for a Rogue to do it all, how about you specifying your exact misgivings against my specific suggestions.
What would be so brokenly bad about giving the Rogue a couple of extra sneak dice?
What would be so mind-numbingly unfair about absolving the Rogue from having to master the metagame of action currency, and just give him the sneak dice you are already allowed today?
Your words, not mine.
Specifically, it was important to know how the player prioritised their choices: you don't want to invalidate their character creation decisions by undoing the trade-offs that they wanted to make.
Now, your suggestions (This is the bit you're probably inrerested in [MENTION=12731]CapnZapp[/MENTION]) would increase sneak attack damage by 14 points at the current level. Plus the backstab dice which I'm going to arbitrarily guess at averaging an extra 5 pts/round. (4 or 5 d6 at 9th level, split into only 1 or 2 combats per short rest, but often used on crits.)
With nothing stated to the contrary, we'll use basic 5e assumptions: The Fighter and Barbarian are probably using weapons that average a few points higher than the Rogue's and have two attacks. GWM is worth up to +5 damage on each of those attacks. Everyone has +5 ability bonus.
Rogue damage per round would be around 46 (d8 +5 +9d6 +5). 50 if they dual-wield as would be optional. Getting additional attacks in from reactions would be highly useful with these changes, but we'll assume that they benefit the other classes as much.
Fighter DPR is going to be (2d6 + 5 +5 +5) x 2 or around 44. Chances to hit should be about the same.
So: it kinda depends upon whether you regard the Rogue dominating the combat as well as out-of-combat situations is what you're aiming for. I'm guessing that it'll make the Rogue player happy. How the players who actually build their characters for the purpose of combat will feel about it, only you can tell.