A person entering the garden of a property has an implied licence to be on the property provided they intend to act lawfully (eg a postman) and not unlawfully (eg a burglar).
The person is deemed a lawful visitor.
The implied licence can be terminated by the occupier of the property. The tenant asking the visitor to leave would terminate the licence.
At this point, the lawful visitor becomes a trespasser if he or she refuses to leave.
The tenant should take out their phone,mand take photos of the trespasser, and tell the trespasser that they will be sued for damages for trespass, and the photos will be the evidence that will win the case.
Note: if you own or rent a property, and your children still live at home, and they are over 18 so you are no longer legally responsible for their care and custody, if you ask your children to move out and they don't go, they become trespassers, and you can sue them.