Ngl, I feel like there are better TTRPGs for this particular application, but either way roleplay is a great outlet for therapy.
Yeah something lighter on rules and RNG would probably be more helpful. It might be difficult to conquer social anxiety for example when you need to roll charisma to make people like you.
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.
