This sounds like a desync issue, possibly due to a high network latency / lag. It may help to close any other applications that may increase latency or use bandwidth (vpn, browsers, chat apps, etc).
If the person with the issue hosts the game, do you and the other friend have the same problem?
Does it help (at least temporarily) if the host opens the party management screen and switches which person is controlling which character, and then swaps them back?
Did your friend try disconnecting and rejoining?