If your organisation is in the habit of recording the time spent on every task, then ensure that there is a time recording allocation for interruptions or working for others. Otherwise, ask the employee to make an informal note of how often he / she is interrupted, and for how long.
Once you have the evidence, you can use this to take action with your employee (for accepting the other guy's work) and / or with the other guy (for consuming your team's time), based on hard facts. Anything else runs the risk of opinions and interpretation getting in the way of the truth. And the facts will help you if you have to escalate the issue to your own manager at any point in future.
As a manager, you need to decide on a course of action that will get the result that you want - so make sure you know what the desired result is before you tackle anyone. Then use the facts as the basis for presenting your argument - and keep it objective, not subjective.