I just added the fourth point about blocking unknown numbers. If your buddy is in a situation (e.g. certain type of job) that makes this impractical, then I'm afraid there's no better solution yet.

Edit: Actually, Google Pixel phones have an automatic call screening feature using the Google Assistant: https://support.google.com/phoneapp/answer/9118387. Demo: https://www.youtube.com/watch?v=jWap-nPQUO0. The privacy trade-off is that Google screens all of your calls, and the feature does not work with custom OSes, but some people might find this to be worth the added convenience. I'd love to see a FOSS implementation of this after a free voice recognition solution becomes available.