I don't get you. You chose a secure messaging platform that you don't trust?

He's using a messenger app with lower security than he'd like, because he can't use his preferred platform.

Without having a smartphone, using Signal wasn't an option (at least previously).

Sadly, almost everything involves tradeoffs.

He (understandably) prefers to use a tool which doesn't require a phone, over one with a better security model.

The desktop version never required a running smartphone (one might even be able to complete signup using the app in an emulator and a dumbphone or a VoIP provider to receive the SMS, not sure)