Hmmm, legally she's entitled, just like any Tom Dick and Harry from the EU is entitled to come here and claim benefits etc despite not contributing, but I think the point of the OP is, should she be entitled?
IMHO, no, because I don't see this as any different to EU citizens who abuse the system. Yes they are doing it intentionally, and this old lady isn't, however, through her own choice (in living her life in Africa) has never contributed Tax to the UK, and therefore shouldn't be entitled to any of my taxes. It's one thing for someone to be on disability etc and therefore not contribute in that manor, it's another for someone to not even live in the country for any of they're working life and turn up (through unforeseen circumstances in this case) and start claiming a pension.
I know what I have said is harsh, and without compassion, but I believe unless you have contributed to a country, then you have no rights to benefits from it, unless you are a school leaver looking for a first job. Maybe even if you have lived outside a country for X amount of years legislation should state that you have to accept that countries citizenship and give away your "birthright"?