This could become quite complex - depending in part on whether she is a British or Irish citizen, how long she has lived out of UK, why she is returning, whether she intends to work in UK etc. It may be that she would be treated as not having a "right to reside" for benefit purposes, in which case she would not have any entitlement to benefits.
I don't think this is a query which can properly be answered here - you need to get advice from a welfare benefits specialist - e.g. at a CAB or other free advice service.