Hi,
Not sure if your mum is religious or not, but an Italian friend of mine used to go to the (Catholic) Italian Church in London. It's in Central London, but she may well meet quite a few people in her age group there, so might be worth a visit? http://www.italianchurch.org.uk/
There is also a website called Italians of London, they regulaly organise get togethers and large Italian gatherings in bars and restaurants, but for younger people, so not really suitable for her.