HMRC and self assessment nightmare

[Guest]

A member of my family returned from 3 years travelling in Australia and New Zealand in October 2019, they are not employed. A couple of months after she left, mail from HMRC was forwarded to me from her old address. After managing to contact her on Facebook I wrote to HMRC on 16th December 2016, explaining that she had no fixed address, but was happy for me to deal with the matter which was supposedly underpaid tax, but mentioned that having left the UK in October I suspected the matter would to some degree resolve itself and asked for their guidance. They phoned and told me to leave it until she returned. On her return she contacted HMRC online and no* has been faced with a fine for not completing a Self Assessment, Plus the tax outstanding. She has offered a number of times to pay the tax, but this was refused. However she is contesting the Fine for Self Assessment as she has never been self employed nor has she previously registered for it. HMRC are not listening and keep trying to say other letter were sent. If they were, they were not sent to her, care of me me, as they knew she was in Australia. Any ideas on how to proceed here? Thanks.