matrix structure issue

Hello, this was the question I got in DEC 2020 exam. An audit firm employee reported senior’s unethical behaviour to the CEO. One of the issues mentioned was that the manager will assign works to the employee even though he was doing other works for other senior manager, which caused the employee unable to complete in timescale, and led to customer complain. The other senior manager did not say anything to this.

Could you advise any solution to this please, I can only identify that its the problem of matrix structure, but have no idea how to solve it. Thank you.