Keeping a diary for your partner seems a good idea, but based on the evidence you have indicated I believe (as other including Ankou have said) that this action by the employee should be subject to a disciplinary interview. This is a necessary first step, I'm afraid and those that have jumped in and said dole out a verbal warning are wrong - you have to first establish the other side of the the story by talking to the employee. Then, assuming he has no good reason for his action, you can decide whether to issue a verbal warning. It is good practice to confirm a verbal warning in writing afterwards, by the way - yes, I know that sounds stupid - but it avoids the employee saying alter 'you never told me that'.