If they don't have Signal you could get them to either use Whatsapp, or only use the encrypted conversation feature of Facebook's Messenger.

They should have one or the other already installed if they're complaining about "another messaging app".

Very true, i do default to WhatsApp quite often, but not being in FB's eco system is something I'd like to avoid (especially given the the privacy bargain that comes with both)

This works fine until you get someone who only uses iMessage, and you use Android.

Exactly what finally convinced me to add whatsapp.