hi ya, As they were joint owners of the property and there was no declaration of trust drawn up when thye brought the house, technically he could claim half of the house and ask your partner to buy him out.
On the other hand if your partner can prove she has paid the mortgage all this time and he hasnt she can argue this in court.
I would tell her partner to change sols and get new solicitors to get this sorted.
He should only take about 6 weeks to sort out.
PS i work in the conveyancing department at a solicitors