What is the best way to handle a key employee (star performer) whose personal life is messed up and is breaking him but not affecting his performance?

I belief he is practical and professional as well as able to maintain emotion. It is a good sign, he is performing well. But it does not mean that he will be able to perform in the long period. People is human being and he will response the emotion in any time. If he does potential asset will be a bad asset. For that reasons, he needs social linkage to protect his problem and supervisor can talk with him privately and can offer counselling.

It seems that the person is able to manage personal and professional life pretty well. It depends if the person needs help to sort out his personal issues or it would impact him adversely. Mostly such kind of individuals feel they are in control of their life and hence would not appreciate much intervention. However if the person is ready to speak then you need to just listen out his issues, help him find internal resources within his family or friends that can help him sort out the problem and thereby help him find a solution. You need to equip him to find a solution for him rather than give him a solution.