The only reason she would be prevented from renewing her licence, is if she developed a 'condition' that would warrant her doctor writing to DVLA to refuse her a licence. If nothing happens, I think she can just self certificate to say she is still 'healthy'. My Mum is 85 and still drives and thats what she does. Obviously if you think she is a bit 'iffy' at driving, you might want to have the 'isn't it time you gave up driving' chat with her, as most people don't want to give up or don't realise how bad they are (reactions, vision etc), though she may still be perfectly competent of course...just saying. This page may help you
https://www.gov.uk/renew-driving-licence-at-70#before-you-start