I have the same problem on my playthrough with my girlfriend. We are at the Underrealm and fought the Duergar at the edge of the lake and we think since then her character can't interact with anyone. We suspected it was related to the Silence Spell because that was used by the enemies there. We also tried to cast it ourselves afterwards but it didn't help. However, when she steals something, she is spoken to and can answer.
