Is your Mum’s house, and the others, council property? If anyone one of them is then the alley is probably owned by the council and the council should be informed of his actions and he should be told to remove it.
If all the houses are privately owned then your Mum and her other neighbours should get together and tell him to take it down or at least remove any method by which it can be locked. He doesn’t own the passageway or have any rights over the control of access. The passageway is probably shown in their deeds as common property.
If he fails to do it they should be within their rights to remove the padlock and or the gate, provided they return both to him.
Failing that he should be reported to the council for anti-social behaviour.