That works to your advantage as removing a 'tenant' could make life awkward.
As Mrs Pegasus says, go to a solicitor and make her an offer. If she refuses to play ball, assuming the offer is fair, you can force the sale (but I don't think you have to put the house on the market).
Ring around the soliticors and see if you can get one to give you the first half hour free.
Just a solicitors letter might prompt his ex to sit up and negotiate - but if she's anything like my ex, be prepared for a fight.