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Has your son notified the Utilities that he no longer lives there and given them her details?
How much is the property worth and how much is outstanding on the mortgage and the loan?
As a joint owner of the property your son can force the sale via the courts if necessary
The starting point is to write and ask her to agree to marketing the house - if she will not do so then he can apply to the courts using the Trusts of Land and Appointment of Trustees Act
The process is described here
In the mean time he should transfer all the utility bills to his ex's name - since she is living there, and since she has moved a friend in he should inform his ex that he will no longer pay half the mortgage - this will not change his rights on the property
I hope that this is of assistance - please ask if you need further details