Oh dear - I assume that they didn't do anything legal when they split up, like getting a legally witnessed separation order? That would have laid out whose responsibility it was to pay for what, who owned what, etc. He must be laughing, he's dropped her right in it - and we don't know the circumstances of their split.
She needs to listen to her solicitor - if it goes against him, she could apply for him to meet the costs of the case, I believe.
Has she told the building society what's happened, they need to know otherwise they'll come to her as joint mortgagee.