Your mum should put in writing her dissatisfaction with the ERC and advise Aviva that she is thinking about asking the FOS to review the matter. If your mum went to the FOS, it would cost Aviva a £500 case fee and they might be anxious about the complaint being upheld. If they have a complaint upheld, it could open the floodgates to other people making similar complaints. The FOS do publicise some of the cases they have reviewed, so financial companies are often willing to look into complaints and make some compromises if they feel it is warranted.
That is the way I would proceed, but if Aviva don't compromise, your mum may have to wait many months for the FOS to review her complaint.