Talk up how much better it is as an SMS app. I've been a signal user for years; with a great deal of effort required to become so (tracfone); and never sent a secure message with it.
"No one can censor you if they don't know what you're saying"
From their app store of choice or directly from the signal website?
How do I get them from asking about it to doing this? "App store" seems like a dead-end answer...