Since she'll be a light user there is a good deal from O2 for up to 8Mb broadband for £7.34 per month (if you're already an O2 customer) or £12.23 per month if not which is still quite cheap. Plus you get the first 3 months completly free so the total for the year will be:
O2 customer: £66.06
not O2 customer: £110.07
http://broadband.o2.co.uk/home/index.jsp
As for BT, its a bit more expensive, at £15.65 per month for the cheapest option so your mum is better of going with O2 to save a little bit of money, plus get a better service at the same time.
total for 12 months (12 month contract): £187.80
total for 12 months (18 month contract): £164.19