A webcam or a Ring doorbell are good ideas, but it won't help if they open the door and start talking to them. Somehow you have to get them to not engage these people. AARP has magazines and newspapers they send to members that talk about many of these scams and warn seniors to not fall for it. If they're not members of AARP maybe they should join.

I know seniors are gullible when they get older, but you have to keep reminding them to not talk to those people.