https://bitchat.free/

do they have access to google play?: https://play.google.com/store/apps/details?id=com.bitchat.droid if google play is banned or they don't want to use it, they can get from github: https://github.com/permissionlesstech/bitchat-android/releases/download/1.5.1/bitchat-1.5.1.apk they if they apple ...

Bitchat.free lists the store & GitHub links for bitchat. On Android, once one has the apk, they can use quickshare to send it to other nearby phones without needing the Internet. https://support.google.com/android/answer/15728591?hl=en