The police should tell them that alliens are afraid of having their picture taken and that if they (the couple) will take a picture of the "alliens" it would help solve the issue.
This sounds exactly like my grandmother as she slipped into Alzheimer. Their family needs to take action. I don't know how many times we had to go chase away "circus people" that were tied to her trees. Fortunately, she only reported it once to police before we could get to her house. I hope someone takes care of this couple.