The link above should help but if not you can contact Acas who can help - especially if they are trying to pull a fast one! ☺️ https://www.acas.org.uk/contact
Unless she has signed a new contract her holiday should remain the same ? X
https://www.gov.uk/calculate-your-holiday-entitlement She can use this to calculate statutory entitlement - if she uses it as starting/leaving part way through a year to give the Nov - April bit that should work. Alternatively if it's 1st Nov to 1st April for example that's 5 full months so if she takes her entitlement for 12 months and works out 5 months of it - so e.g 24 days for 12 months would be 2 days per month so for 5 months would be 10 days (I used 24 days just for easy numbers) Hopefully that helps! X