We know of a dominant African Grey that was cured of feather plucking by placing its cage above the rest of the flock. Any addition to the bird community requires a re-establishment of each bird's position in the avian hierarchy. After quarantine, always place a new bird in a separate cage beside the cage it will eventually inhabit. Once they all appear comfortable, introduce the new bird into its new home and monitor the introduction carefully. It may be claustrophobic.
Many parrots grieve over the loss of a mate.
Introduce either of these to your bird slowly. Any addition to the household requires a bird to adjust its place in the hierarchy of its extended family. Perhaps your bird was preparing for another clutch when you disrupted its plans. An increase in activity can be very disruptive to a bird that is used to having privacy. Too much direct sunlight can cause your bird to overheat.