If she's not at her home address that IS a change of circumstances and
ANY change of circs MUST be reported even if it results in no change in entitlement.
If someone is outwith Great Britain for more than four weeks, the Housing Benefit can be stopped but there are instances where it can continue for a longer period.
If you're away in Great Britain, it can remain in payment as long as the absence is less than thirteen weeks and the intention is to return.
Having said that, due to current circumstances, those regulations might have changed.
Either way the change of circs MUST be reported and whilst reporting that, she can ask about how long she can be away for.