My parents (father since died) put money into Halifax 'Money Fund'. They were told they would not lose, but would benefit. This then apparently changed to some form of 'investment'. I noted in January 2019, that a letter had been sent just under 3 years previously, advising that this was not the best place for their money. I made contact (after taking funds on my mother's wishes) and complained, as it was under their 3 year 'cut off'. I was told that this was a second letter, and that the first (sent over 3 years ago) was the one which mattered (we have no trace of this - asked for a copy and they sent a template). What can we do, as this money may as well have been under their mattress? It was originally mis-sold.
I would imagine that this is quite worrying for you and your mother, and you are within your rights to ask Halifax to investigate your concerns, and the initial advice that was given to your parents.
Investment advice would normally require the adviser to produce a suitability letter, to document the reasons why the investment was appropriate for your parents, and this letter should document the risks and features of the plan, and why it was deemed as suitable.
You can ask Halifax to investigate if the original advice was suitable, explaining your concerns that your parents were under the impression that they would not lose money from this investment, and the letter that was sent by Halifax advising your parents that it may not be the best place for their money.
Any communication that they sent to your parents should be on record, and you are able to request copies of this.
It is unlikely that a savings product would have changed to become an investment without an advice or non-advised process, and it may be that your parents were not clear that they were taking an investment product. If this was the case, then they are within their right to ask Halifax to investigate this, and you can ask them to provide you with a copy of the documents that was issued to your parents.
This should be your first step to help resolve this for your mother.
If you are not satisfied with the response that you get from Halifax, then you can take your complaint to the Financial Ombudsman Service within 6 months of the final response from Halifax.
I hope this helps,