Give the neighbors one chance. Perhaps b/c of their background....they have no clue how a *neighbor* behaves.
Next time they are out....go over .... with a marrow bone and dog cookies [presumably for the dog].
Is there a chance in the future your frail mom will be the one going to the mailbox? If that is the case...I would take this very seriously, but I would still approach the neighbor in a friendly way. Give them one chance to fix things. One.
And don’t let mom go the mailbox.
hot dogs with exlax over the fence