Since they're not technical, their natural self-defense against this kind of this has trouble functioning online. Talk to them about real world analogs to pop-up ads that look like virus scanner alerts, talk to them about people pretending to be someone they know. Hell, show them the movie "Catch me if you can" and explain the same psychological tactics get used online.

Get them to consider "what do I really know, and is this too good to be true" before they talk to anyone, click a link, or buy anything online -- and they will be fine, if their judgement is otherwise unimpaired.

If they're very late in life, or otherwise have trouble with this kind of thing in the real world, there is unfortunately not much you can do to help them, other than force them to only use pre-installed apps on a tablet that you've selected, like you might with a small child. If they're not willing to do that, then it's unfortunately on them. I've had this experience both with my grandfather and a friend who lives with a brain injury, who are responsible for themselves but don't have the judgement necessary to realize how impaired they are.

And of course, make sure the computer is auto-rebooting to get updates, and they know to call you if they get a virus / malware popup they think could be real.