There are multiple ways to inform a specific user of some kind of event. However there no standard or build-in way to achieve this in Mendix.
The following are 2 examples that we have applied in the past to achieve this behaviour.
1) Create an in app message system like the one used in Mendix Support portal.
2) Send a notification mail to the user based on a registration system that allows the user to register itself on certain events.
Both of these notifications rely on the fact that you need to create some sort of trigger mechanism that is hooked on before/after commit/delete and checks whether the object is new, has been changed or it has been deleted.