Yes of course you are, you are the householder (or rather your Mum is). This happened to me, a bloke down the road opened a mobile phone account but they had my address and he defaulted. Open the letter, write a very stiff note to Vodafone and the solicitor or whoever is sending the letters (ring them up too, but confirm it in writing) to say that this person doesn't live at your Mum's house, there is no association, and can they stop writing these letters. Say you will consult your own solicitor if they don't stop. Don't let your mum be frightened by reference to court or bailiffs, it's not her debt, she's not liable.
The trouble is that just mailing them back "return to sender" doesn't tell the sender that the person doesn't live there, so they keep coming. You need to take action now to stop this - it's a shame you don't still have the latest one - unfortunately if it's a debt then the letters get more and more shirty if they don't get answered.