Depends on the contract of employment. However, minimum holiday entitlement for anyone is 5.6 weeks pro-rata the days/hours they are contracted to work.
holiday entitlement is what it is, however I would strongly recommend the disciplinary route for that employee!!
you don't say what the time off is for - sickness?? You do have to be careful though and it is a mindfield but there are ways - call ACAS for more, its a free phonecall and they are very helpful.