DataContextChanged on a TreeView to trigger a method?

Question

I have a User and a Manager, and the user has a To-Do-List with tasks that have been assigned to them. The manager has Users names as labels on there main screen and below that a number of tasks that are in their To-Do-List.

My objective is whenever the Users gets assigned a task in their To-Do-List, it instantly triggers a method at Runtime on the manager screen that counts the tasks in the users To-Do-List and updates their label on the managers page.

I don't know where to start on this task, hence why I am asking this question.

I am not asking for all the code or any for that point just to be told how I can achieve what I want and I can work and figure the rest out for myself.