The Signal messenger and protocol.

I use Signal, that said:

Are they very good friends if they won’t install a single — free — app to stay in touch with you?

Is someone a very good friend if they won’t communicate with you because you won’t use their preferred app? That’s the real question you’ve presented.

You don’t have to compromise your values due to peer pressure.

You seem confused, if you’re trying to talk someone into using your preferred app that they may know nothing about, then you’re the one engaging in peer pressure, not the other way around.