As you have said, this is a joint mortgage so the debt and ownership of the property is 100% to both parties.
If the property is sold, whether for profit or loss then this falls to the responsibility of the applicants...whether they are resident in the property or not. just because the girlfriend has walked away she is still as liable for each and every payment as your son...although it is a little harder to prove she is not paying and the bank will not care if the direct debit arrives as it should. The downside for the girlfriend is that if your son sells the house or gives it back to the bank and makes a loss of £20,000 then this debt is still jointly owned by both and the bank will chase both parties and not just the person who has been making the direct debits. She can walk away from the house but she cannot walk away from the legal contract she made with the bank....even though she has not paid a penny toward the house for 2 1/2 years. The contract still stands until it is renegotiated or the debt is paid off.
So far as the rent, she has decided not to pay into the mortgage, why ever would she think that she is entitled to any rent payable, especially if the rental agreement states your son as the landlord and not her. This would then make the legal financial contract (Assured Shorthold Tenancy (AST) would be between your son and the tenants and not her. If she wants to start paying back into the mortgage and make some back payments for what she may owe your son then you may decide to share, but until then I wouldn't worry.