As part of this discussion, it can help to explain the impact the employee’s behavior is having on the team.
When dealing with a previously high-performer, start with affirmation of good work but move on to state the facts clearly early in the conversation.
It is best to be direct and ask why this is happening, but also make sure to really listen to what the employee is saying without making snap judgements on the behavior.
In a situation like this, it is good to help the employee accept responsibility, and also help them to come up with a plan to change the behavior. This plan should be documented in some fashion and then a follow-up discussion to make sure that the plan is being adhered to should be scheduled.