Joint tenancy - one not paying

I am posting this on behalf of my friend whose son bought an apartment with a friend under joint tenancy. The son's friend lost his job and stopped paying his portion of the mortgage and my friend's son continue paying the full amount for a short time. However, he couldn't afford to keep up the repayments on the mortgage so let it go into arrears. (also in negative equity)

Now, almost 3 years later, well in arrears and with a restructured loan agreed with the bank for another 2 years...

Joint tenancy - one not paying