I hope Barb is sane enough to not feel guilty about another person's actions, but many aren't. The problem with the ideal scenario you describe is that Barb and Jenelle were in a toxic relationship long before David showed up. There is no way Barb could "fix" the relationship now . Also, if suspicions are correct, there is a significant drug use problem with Jenelle and/or David. In that case, no healthy relationship can be established.
If I were giving Barb advice, I would recommend counseling for herself and Jace, and Al-Anon for her so she can learn the limits of what she can control. The only person Barb can change is Barb. I would also recommend she stay away from any toxic, volatile situation involving drugs and weapons, which means the situation on The Land.