Accrued holiday and sick leave, general questions

[TeenTimesTwo]

Not sure if this is the right section.

DD, 19, has gone on sick leave - 2 months so far, failed phased return to work, so likely to be another 2 months. Back issue since Jan(!), physical job, no diagnosis yet.

She has at least 1 week's leave accrued which she won't be able to take by the end of the leave year at the end of Dec.
What should happen to that - does she just lose it, or should it be paid off to her?
Also they 'need' 3 days leave over Christmas, how does that work when off sick?

I don't want her taken advantage of, given that a) her injury is at least partly their fault imo, and b) they were under paying her (i.e. less than legal minimum) for 2 months until we found out and got her to query it.

"Any statutory holiday entitlement that is not used because of illness can be carried over into the next leave year. If an employee is ill just before or during their holiday, they can take it as sick leave instead."
www.gov.uk/taking-sick-leave

Sounds like the leave should be carried over to next year, although some employers may pay it.

Thanks.
She may well end up having to leave if we can't get it sorted, in which case I guess they'd just pay it off?