Easy! Just replace their usual SMS app with Signal, and then every contact they have that does use Signal is private and secure!
Oh. Wait. That’s exactly the functionality that Signal removed in their effort to ensure that Signal is never widely adopted…
And that irony now is that messenger on Android is RCS compliant and currently has this exact functionality, except it’s less trustworthy.
Once again I’m using one messenger and everyone else who’s using an RCS messenger gets encrypted, but SMS (clearly marked as such) is a viable fallback.