As bionicjoey said, something more rules-light, and I would add: less focused on combat. Of course, not all D&D encounters need to be combat-focused, but that's where most of the rules lie.

Another concern would be theme, so something like Blades in the Dark or Candela Obscura would probably make a poor choice, despite being rules-light, story focused games.

Savage Worlds is a bit simpler, with a generally empowering tone. Or perhaps Mouse Guard, for a more cozy theme.

Honestly, for a patient with no experience with RPGs at all, I would argue for a very simple "just roll dice and see if you succeed" non-system. Though I can't deny there are potential social benefits to learning D&D specifically, if that's the goal.

I'm not an expert on Psychology nor TTRPGs, so take my opinion with a grain of salt.