Pensioner's Wife Spouse Visa Extension

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Dear all,

My father is a British Citizen. My mother came in UK by spouse visa. My father was working full time when he first applied for my mother.
Now my father is a pensioner. My mother's spouse visa is going to expiry in 8 months. She need to take preparation to extend her visa. But my father's income from pension is only £928.6 per month, which is £11143.2 a year. My mother is also not working.
I need your help on the following questions.
is there any exemption in the immigration rules for pensioners wife? if yes what is the requirement.
can my mother extend her visa even if she does not meet the financial requirements? if yes then How?

Thanks