Get all the users that are managed by a targeted manager
No simple select exists because MySQL does not support Recursive CTE queries like SQL Server.

This being the case, I wrote up some Stored Procedures to run iterative techniques for gathering children of a parent out of a hierarchical data design.

Here is my old post from Oct 24, 2011 : Find highest level of a hierarchical field: with vs without CTEs 

I have three Stored Procedures


GetParentIDByID
GetAncestry
GetFamilyTree


In your particular case, you need to do two things


Change GetFamilyTree to gather all users from the user table under the target user
LEFT JOIN the result from GetFamilyTree with the customer table (I suggest LEFT JOIN rather than INNER JOIN in the event a user does not have customers, has not been fired, and it still in this hierarchy).