Hello Jessica,
First, I want to say I’m sorry he is going through this, its intense! As for witchy advice, the first thing that came to my mind was psychic self defense. A lot of protection spells and rituals are aimed towards energetic/spiritual protection, but as all things are ultimately manifestations of energy, I feel like they also help with other areas of protection as well.
I am glad to hear that your husband cut ties with this person. Did he do a spiritual cord cutting as well? This could help break things off even further. I feel like the first step is to do a cleansing/banishing of the situation, followed by a protection spell (which there was a wonderful Banishing Ritual & White Light Protection posted about this on the forum by the way! There are other ways to do this without performing a ritual, and are more along the lines of energetic transmutation. I found a great article that goes into how to change negative energy into positive, useful energy at /Converting Negative Energy into Positive Energy | Mysticsense It even talks about how the length of the relationship affects the strength needed for your spell to be successful. Hopefully it will help him move beyond the stress and remnants of this drama, and allow the negativity to be transmuted into someone positive and uplifting. It takes time, and the steps will likely need to be repeated, but it is possible to overcome this.