x2 on the everybody talking suggestion.
You might try just talking to the people involved in the situation. Her, her husband and however much you can get your husband to talk about it. (Don't push too hard, some people are OK with situations more so if they don't have to talk about it.) Talk about possible boundaries and scenarios and worst-case and best-case. A pre-plan can lead to fewer landmines down the road.
Good luck. You must feel incredibly insane right now.