can he change the locks?

[porridgeoats]

My son bought a property with a good size deposit. He moved to a larger property and put his partners name on the deeds. She has been borrowing money and got herself into debt with different companies and a money lender and has threatening mail from them. She has been hiding all the mail including my sons income tax and has got him into debt as well. He started finding mail hidden in places under the mattress, the carpet etc. She has moved out leaving debts but keeps coming back for the mail when he's not there. He's worried she'll take his mail. Can he legally change the locks?