If your friend has irregular hours / flexible contract as a carer then her holiday entitlement will be based on the hours she has actually worked.
https://www.gov.uk/calculate-your-holiday-entitlement
Have a look at this leave calculator, your friend can select the 'casual / irregular hours' option, then enter the number of hours they have worked within the leave year (most businesses seem to do their leave allocation based on financial year April-March, but they may do it Jan-Dec).
The calculation will show the basic level of holiday entitlement.