What's your/her definition of 'south'? Do you mean the southern half of the country (south of Yorkshire, say) or do you mean 'south of London'? If it's the former, Bury St Edmunds (here in Suffolk) is a delight:
https://www.visit-burystedmunds.co.uk/
Saffron Walden, in Essex, is very pleasant too:
https://www.visitsaffronwalden.gov.uk/
as is Maldon:
https://www.visitmaldon.co.uk/
At one time I wouldn't even have considered suggesting Ipswich (where I was born and brought up, and which is now the nearest major town to where I live) but these days it's been revitalised and has a great deal going for it:
https://allaboutipswich.com/
If your mother wants to stay close to the coast, Felixstowe is worth considering (and a great deal more affordable than Suffolk's 'prime' resorts, such as Southwold, which is an absolute delight but where a 2-bedroom terraced house can set you back over a million pounds):
https://www.visitfelixstowe.org.uk/
South of London (just!), Whitstable is very pleasant and, because it's just down the road from Canterbury and within easy reach of London by rail, within range of lots of places to visit and things to do:
https://www.canterbury.co.uk/info/37/whitstable
(Houses aren't cheap there though!)