Samy was in a coop dungeon battle with me about 3 hours before the new season started. During the battle her game messed up and she has not been able get on after that.
Are you trying to login with their account under your PSN, which already has your PQ3 linked to it - Or are you also using their PSN to access their account?
As you can only have one PQ account linked to one PSN account at a time.