Hi, I'm Pete. Welcome to JustAnswer! I'm reviewing your question now, and will post back with your reply ASAP.
If the app is working with all other players then the problem is with the affected player.
Have you tried contacting them through other methods to check if their app and device is ok?
You stated the problem only happens with one person, therefore the issue is not with the game.
Are you sure that the other person hasn't blocked you?
Let me check with the other techs if they have any idea what is going on...