Hello my name is Alex and I will help you with this. Please note that I am a working solicitor and may be on and offline. As such you may not get an instant response when you reply, but rest assured I will be giving your question my immediate attention upon return.
I am sorry to hear about the situation you Mum finds herself in. Although I don't think it would be illegal to purchase the house for the same price as it was when the mortgage was taken out, I think you would have great difficulties in getting the mortgage lender to remove their charge from the property unless it was sold for the full market value.
This type of mortgage relies on the increase in the value of the property to pay back the capital amount, which during periods when hose prices don't increase greatly can be a good deal for the borrower however the opposite is true if house prices rise dramatically.
If you feel that you father may have been mis-sold this mortgage there is a Shared Appreciation Mortgage Action that may be able to provide some advice on options open to you. Their website is at http://www.samgroupaction.com/. You may also wish to speak to a solicitor that deals specifically with this type of matter, you can search for a specialist solicitor on the Law Society website at http://www.lawsociety.org.uk/find-a-solicitor/ .
If this answers your question then please take a few moments to rate my service as it is an important part of the process. If you need more information then please click reply. Alex