Your neighbor hands you an ultimatum. Cut down those trees or else!
Or else what? Or else go to court.
Wait. They can't do that, can they? If this is a tree vs. view case in any of the towns that have a tree/view/sunlight ordinance, they can, but not quite that fast.
All the tree regs in all the towns with ordinances provide a step by step method for resolving your dispute before going to court. Sausalito even has a Tree Committee. All the towns require informal discussion first, followed by an attempt to mediate.
And by the way, the one initiating the mediation doesn't get to choose the mediator. It needs to be done mutually. If they suggest one, don't agree until you've had a chance to review the person's bona fides and look into some alternatives.
If you don't want to mediate, arbitration is an option with a neutral arbitrator, (professional or not), or with the Tree Committee in Sausalito.
Don't want to arbitrate? You can get an informal opinion from the Tree Committee, and if you get invited to appear, you better show up, or it will be your burden to overcome their opinion in court if it goes against you.
Make sure you get your own consulting arborist to evaluate the situation and a competent tree attorney to represent you.
A note on mediation: A while back, I asked all the towns with tree/view/sunlight ordinances in Marin County if they provided a list of neutral mediators for tree disputes. None did. That's too bad, because it's hard to know on your own what mediator has experience with your kind of case. Your arborist may have some ideas. But remember, the mediator must be mutually decided on by you and your neighbor.
Consider one that has an expert consulting arborist/tree risk assessor as part of the team. That way, you and the neighbor can save money, save time and get a neutral scientific evaluation of the problem. You just might save your relationship as well.