Hi Fal,(posted 7594 days ago)Very tragic and traumatic when things like this happen - lenders claim to be symphathetic but never demonstrate it :-(
Advice is as other posts but I wouldnt necessarily seek legal advice unless court forms or a land registry charge notification arrives through the letterbox. Some solicitors tend to 'put their feet in it' and as an agent representing your mum could re acknowledge the debt for another 12 years.
Incidentally some temporary relief may happen soon - the rumour is that Eversheds are leaving the mortgage shortfall arena very shortly also follow the info on this site - SARN the lender - see what they have on your mum.
Good luck and best wishes.