Assuming you are a bristish company...
The legal minimum entitlement to give a full time worker is 4 weeks paid leave (20 days) per annum. These 20 days cam currently include bank holidays (as you are not legally obliged to pay bank holidays) but it is thought that this will not be the case for long and that soon it will be law that employees are paid bank holidays on top of the 4 weeks leave.
As to your query about the part timer, they have the same rights as full timers but on a pro-rata basis. So people who work 5 days a week get 20 days. The person who works 2 days a week gets 8 days (20 divided by 5 x 2).
The issue in your specific situation is calculating which bank holidays she will be eligable to be paid for - for example, if she does tuesday and wednesdays then nearly all bank holidays won't effect her. As such, in her situation I would say the easiest thing to do would be say her entitlement is the 8 days (she is entitled to these) and specify that bank holidays go unpaid.
See
http://www.acas.co.uk/index.aspx?articleid=806