@radstylix, Youhei (and maybe everyone who has strong enough sixth sense to see ghost clearly unlike that old salary man exorcist back then) probably sees spirit using mind's eye or something, basically like a spirit vision in which they don't really need light to see the spiritual form. Hence it's possible for them to see ghost's innards.
Honestly i'm more surprised that the ghost girl actually have innards, considering that now she is a ghost, she doesn't exactly in need of "required-for-living" inner organs.