@WickYou cannot really heal a dual nature ticking time bomb
Considering his mind was never normal to begin with she cannot heal to normalcy.
Being a hybrid of angel and demon it's like his brain is torn in half
Best real life example would be multiple personality disorder (or DID) he has two 'alters' which occasionally take over the main being demon and himself. Angel is there but he doesn't really take over
The rest is because of the warring natures. One pure good the other pure evil. This inner turmoil leads to the issue of going insane and becoming a husk like what he is now. Since he has retreated internally there is really nothing to heal there because he isn't technically damaged in the brain...his brain is technically healthy.
That being said. To bring him back you first have to pull one of the alters then convince them to go back and pull up Cupid. This will return him to his natural state but it does not really cure the full issue. It just temporarily fixes it.
For hybrids there is no known cure...most die in their 20s from starvation...so he's a unique case being living at his age.