When someone has bumped into the corner of a wall and caused damage to the finish, there are some preliminary things that must be done before patching it.
First be sure that all loose drywall compound is removed. Then add screws to the metal corner bead if necessary to pull it back flush. Use a nail set and hammer to bend in any metal that is sticking out beyond the finished surface. File down the outer corner if necessary if there are any sharp points sticking out. Plaster will not cover them up.
With the corner bead securely attached below the finished surface, you can proceed to re-plaster the corner as it was done originally.
You may want to add a plastic or metal decorative corner protector if this corner is exposed to the risk of being damaged again.
Decorative corner protectors are commonly available in most renovation centres and hardware stores.