Does your daughter live alone?
I take it she does not know this neighbour even to speak to.
If she is single, it is probably best to ignore this, unless it happens again, then report it to the police.
If she has a husband or a boyfriend surely he could lightly say to this man next time he sees him in the street., something like, "You know, your bird-watching through binoculars scared my wife, she thought you were casing out our house."
If the man is genuinely either trying out a new pair of binoculars or bird watching, I am sure he will apologise and explain what he was doing
If he is looking at your daughter, he has been made aware that she knows and doesn't like it.
Sometimes I feel we invent problems these days, because we don't know and perhaps don't want to know our neighbours.