If she was not given a written notice (ideally on the spot) and there is no notation on the visa stamp in her passport, I am inclined to ignore the advice and treat it as a normal six month visa.
However, I would also suggest treating it as a warning that her travel history has flagged her up to the Border Force. It may be worth asking her to make an SAR to the
Border Force (which is not a part of the UKV&I, but a separate part of the Home Office) while she is in the UK to see if there are any adverse comments on her travel history to the UK.
It is worth remembering that visit visas can be curtailed by Immigration Officers at the airport if they believe that the visitor is not complying with the
requirements for visit visas (one of which prohibits "frequent or successive visits"). They must of course give such notice in writing, as
Vinny has already mentioned.
PS: She did not give the impression that she was coming to the UK to look after her grandchildren or something along those lines, did she? That may be classified as unpaid work, which is also not allowed by the Visitor Rules.
I am not a lawyer or immigration advisor. My statements/comments do not constitute legal advice. E&OE. Please do not PM me for advice.