Implications of defaulting on my mortgage?

A friend of mine is in a sticky position. She has had her house on the market for over a year, is in negative equity, so therefore cannot reduce the price of the house. If she stops paying the mortgage, and the bank repossesses it, what would the implications be? Obviously they would then sell it, but would she be liable for the difference in sale price and outstanding mortgage? She has a very good job, and can afford to pay the mortgage, but needs to move due to work, but obviously cannot afford the possible �20,000 loss on the house as she owes what it is on the market for. Any help would be greatly appreciated, thanks. We are in England.