Mac monitoring software?

Hello all,
    We have a bit of a difficult situation, we have a Mac that was used by an ex-employee, but his father is a GM of the company. He will be coming in over the weekend to do some work for his father, but has been known to do far too many personal things during his time here (like looking for new jobs and submitting resumes, just generally not doing anything productive at all) and is going to be billing us for the time spent. Is there software we can install that will track basically everything that is being done on the Mac? Free would obviously be ideal, but if someone has used software that does cost and knows it works, that would be good to know as well.

Thanks!