From the directgov website linked above
Holiday entitlements: the basics : Directgov - Employment
The basics of holiday rights
There is a minimum right to paid holiday, but your employer may offer more than this. The main things you should know about holiday rights are that:
you are entitled to a minimum of 5.6 weeks paid annual leave (28 days for someone working five days a week)
part-time workers are entitled to the same level of holiday pro rata (so 5.6 times your usual working week, eg 22.4 days for someone working four days a week)
you start building up holiday as soon as you start work
your employer can control when you take your holiday
you get paid your normal pay for your holiday
when you finish a job, you get paid for any holiday you have not taken
bank and public holidays can be included in your minimum entitlement
you continue to be entitled to your holiday leave throughout your ordinary and additional maternity leave and paternity and adoption leave
So first thing is yes they can include Stat holidays. Does she work 5 days a week? If so she should be on 28 days not 25.