I haven't heard of a roadside assistance program that doesn't extend to anyone using the vehicle that has been given permission to do so. While most companies prefer any driver using the vehicle to be listed on the policy, they do have previsions in the policy that extend to infrequent users.
Although I am not a Farmers insurance agent, my understanding of Farmer's Roadside Assistance program is you can either call their roadside service while your vehicle is inoperable and they will tow you up to $150 or you can be reimbursed for your troubles. However, this is all dependent on having towing and roadside on your policy.
I would recommend speaking with your insurance company regarding their specific rules. But, given him using the vehicles often, I would imagine they will require him to be an added driver to the policy.
If you do not have a rideshare endorsement on your policy, they might deny the claim. Insurance companies like to know all the risks they could potentially be exposed to and ridesharing presents a big one.
Fault can only be determined by the insurance company. So, in this situation, the only way for the damage to be repaired would be to contact both insurance companies and handle it through their claims department.
For New Jersey, a vehicle is determined to be a total loss if the cost of repairs plus the scrap value of the vehicle is greater or equal to the actual cash value of the vehicle. Because you know the value of the repairs, your next step would be to determine the value of your vehicle.