I'am back.This time on a different side of the forum

My partner is from South Africa, he was born there.He's got a family visa based on my EU rights.Mean time , I got my British ppt.Everything is ok with us but we were thinking of doing something for his brother or his mom who are still in South Africa.
His mom's case:
She was born in South Africa ,exactly 60 years ago.Her first language is English - not Afrikaan, although she speaks both. Her mom(my partner's grandma) was South African but her father(my partner's grandpa) was Welsh (British).Her father(my partner's grandpa) died in a car accident when she was 3 year old so looon time ago.Based on this:Is she entitled to British Citizenship?
She's got her birth certificate (which mentions his name) and nothing else.
If it is possible, can her child (my partner's brother who is 27 now) get directly at least the right to work in the UK, as the country of his grandpa?
Many, many thanks for your help.
Ukpass123