Halifax is chasing a debt on a house my wife left 20 years ago

Her ex had forged her signature on forms and is still insisting she is liable My wife took out a joint endowment mortgage with her ex nearly 20 years ago. She left the property in 1997 and advised Halifax of her new address, but her ex refused to take her name off the mortgage. The mortgage fell into arrears and the house was repossessed in 2007 selling for £12,000 less than the loan. Now Halifax is chasing her for the shortfall. We also discovered the mortgage had been changed from an end