That depends on perspective but regardless of that, you don't have any information to support this. The man calls the woman "his wife", because he is still married to her. He says "she has since left me" which indicates that they were together at the time of the affair.

As the boss I would wish them both the best and continue to judge the employee based on his merits at work.

Well isn't that fantastic. I would get rid of both of them as soon as possible. Once someone betrays you, they will do it again and again.

It is not clear that the relationship was over. A relationship involves two parties. If both parties are not clear that it is over, then it is not over. This is why formal divorce is important. It marks a clear end to the relationship with no room for misunderstanding or confusion. If they created the relationship with a formal marriage commitment then it is unethical to move on until there is a formal divorce, or at the very least a clear and communicated separation and intent to break the relationship. Now if she did everything she could to secure a divorce and he refused her, then it is a different story, but there is no evidence that this is the case here.

It is no longer just her choice to make. When she entered into the marriage relationship, she made a commitment to her husband. She made a promise. It is no longer just her choice.

This is a stretched analogy, but ethically similar, I believe: If I enter into a contract with a service provider (marriage relationship) and as part of their commitment to me they promise a certain level of service (fidelity), then they are obligated ethically to provide that level of service as long as they are within this contractual relationship. Unless we have written into the contract that either party can dissolve the relationship at any time without notice--uncommon for this kind of contract and extremely uncommon for marriage--then they are in the wrong if they choose unilaterally to end the relationship and fail to meet their committed level of service.

This is why adultery is often illegal. The only argument I can think of for making adultery legal is that we do not want the government this involved with people's personal lives. It may not be in fashion, but I firmly believe there is no moral or ethical justification for adultery.

This order of events is incorrect from the OP.

> Recently, I found out that one of my employees has been having an affair with my wife. ... My wife has since left me.

His wife may have already told him it was over years ago and she literally left after the event. Or perhaps the event caused the divorce. Either way it is an 'affair' from his perspective.

Many people live together for years after it is clearly a failed marriage.

I was also not stating the order of the events, rather the facts of the events. The order is of little importance here. She has left him for this other man. That is her choice to make.