Neighbor Refuses To Split Termite Fumigation Costs For Our Attached Duplex. What Are Our Options? (california)

Location: San Diego, CA
Looking for advice on how to handle a termite dispute with our neighbor.
We purchased one half of an attached duplex in July 2025. The neighbor owns the other half and has lived there for over 30 years. There is no HOA.
Shortly after moving in, we discovered evidence of drywood termites. We had a professional inspection, and the inspector recommended tent fumigation. Because the two homes are attached, we were advised that coordinating treatment for the entire structure would be the appropriate approach.
We first approached the neighbor in September 2025. Since then, we’ve made numerous attempts to coordinate treatment, including in-person conversations, emails, and certified mail. It’s now been over a year without acknowledgment.

Here’s where things get interesting:
Our neighbor initially said she hadn’t noticed any evidence of termites on her side. She eventually had her own inspection performed in September 2026.
I happened to speak directly with her inspector that day. He described substantial termite droppings in her attic, extensive fascia damage, and also recommended tent fumigation.
However, our neighbor characterized the findings as evidence of past termite damage with no active infestation. She also referred to the problem as “your termite situation.”
We’ve shared our inspection report with her and requested a copy of hers, but she hasn’t provided it.

The disagreement:
She says she doesn’t want her home tented because she’s concerned about chemical exposure to her family, pets, and property.
She eventually agreed to allow fumigation as a “concession,” provided we pay 100% of the fumigation cost. She considers paying for her own temporary accommodations and cleaning expenses during fumigation her contribution.
What concerns me most is that she hasn’t proposed any alternative treatment plan or addressed the potential consequences of leaving the termite problem unresolved.
We’re essentially at a standstill.

My questions:
In California, are there laws governing shared maintenance or termite treatment expenses between owners of an attached duplex without an HOA?
Can one owner refuse to participate in treatment recommended for the attached structure?
Would a party-wall agreement or recorded property documents potentially establish shared maintenance responsibilities?
Has anyone successfully resolved a similar situation through mediation or an attorney?
If she refuses to contribute, what practical options do we have to protect their property without assuming the entire cost?
We simply want a reasonable solution that protects both homes and fairly allocates the expense.
Any advice or experience with similar California duplex disputes would be appreciated.