If the metronome bar is not making progress for your friend after connecting to a jam then their audio is not working yet. It may be necessary to disconnect from the jam session and change jammr's audio settings (File | Settings on Windows and jammr | Preferences on Mac).
If your friend is on Windows try changing the “audio system” from “Windows WDM-KS” to “WASAPI” to get audio working.
I can recommend specific sound settings for them if they go to Help | Show log after connecting to a jam and post the technical information that is displayed.
There is an audio setup guide here that may be useful:
https://forum.jammr.net/topic/1987/