There isn’t one best tabletop RPG system, or one best edition of tabletop RPG system. There’s nothing inherently better about using one system over another. The only real difference is your own preference.
Like, Pathfinder, the first one, is a really interesting and robust system. It’s great if you know it, but if you don’t know it there are a lot of pitfalls and trap feats built in, and it can be hard to make a character that keeps up with the curve if you don’t know what you’re doing. Kind of reminds me of Magic: the Gathering in that; lots of options that seem good out of context, but aren’t really. That’s fine for some groups, for others it’s a lot of extra headache.
Does D&D 5e treat your characters as being more robust and capable than, say, AD&D 2e? Yes, absolutely. But like, even that is just the default behavior of the system. That factors in, but a competent DM can run a ruthless game in 5e too, it’s just a matter of shifting the numbers.
If your partner feels like 5e is too much of a power fantasy for his tastes, I’d recommend trying Dungeon Crawler Classics. You generate a handful of random level 0 characters and take like 4 per player into a dungeon that’s an absolute meat grinder for them. They progress, you add new level 0s if you need to, and you keep going. It’s a lot of fun watching your shitty little level 0 farmer grow into a fighter or a wizard or something. DCC is an absolutely ruthless system though, so be prepared to lean into it.
On the other hand, there is still plenty of AD&D 2e material out there. Maybe more than 5e, I’m not really sure. It was pretty robust, though, and less forgiving than 5e without being quite as bad as DCC, but your little level 1 wizard with 4 health can still get one-shotted by pretty much anything.
Of course, you can always just tune these systems to do what you want. Want to play 2e but don’t feel like figuring out how THAC0 works or ever bothering to calculate it? Great! Don’t. Just slot modern AC and hit rolls in. Start AC at 10 and when 2e says to go down, go up instead. Whenever something would make your THAC0 go down, treat that thing as an attack bonus instead. Hey, check it out, we’ve just stumbled into 3e’s Base Attack Bonus system.
Tabletop rule sets aren’t a god for you to worship, they’re a tool for you to make as flexible as you need it to be. Take the pieces you want, toss the pieces you don’t, add your own stuff. That’s how any of this stuff got made to begin with, and it’s how it progresses.
Want to make 5e more ruthless? Design an injury system and implement it.
But don’t just get sad that some people have some milquetoast criticism about the one edition you happen to have stumbled across first. Who cares? Check out some other systems and develop your own opinions and contexts.