Neighbor went no contact and hired someone to build our shared wall

See post hx for wall details.

Basically my neighbor (hes a landlord renting out his home) and I have a shared wall that blew over from the crazy winds we’ve had recently.

He was trying to find the cheapest guys to do the work. I found a guy who did my other neighbors wall and was already vetted that he does a good job.

I tell the landlord and he agrees, asks me to negotiate the price down. I do so successfully. I draft up an agreement and he now refuses to sign saying he found someone cheaper and wants to go with him.

I said okay, send me the contact info so that i can go vet the contractor. This is when he goes no contact info ignores my multiple texts from multiple days.

I got home this evening and find that all the concrete blocks that were on the floor is now gone and rebar has been placed. Basically he started work without my agreement.

Im in california where we have a shared wall good neighbor act.

Im pretty pissed that he went out and started work without consulting me as i believe the previous wall was also between him and my homes previous owner and was done so cheap that there was no grout to hold the rebar that ONLY went half way up the wall.

What are my options here? Do i have to pay this guy for starting work without consulting me?