Re: Hiring uninsured labor
You didn't say what kind of insurance the tree trimmer doesn't have. For the sake of this discussion, I'm assuming he doesn't have any insurance of any kind.
Do you have homeowner's insurance? If you do, which is highly likely, every homeowners insurance policy in the state of California includes a small worker's compensation component. That component is meant to cover anyone you hire to help with household maintenance and tree trimming should qualify. So, if either of these two are injured during the work, you should be able to make a claim and have it covered.
Meanwhile, if the tree trimmer is licensed, you will not be responsible for his injuries. You are supposed to possess a license if you are trimming trees above a certain height, which I don't remember offhand (perhaps 15 feet??). The helper is another story. The tree trimmer must have workers compensation insurance by law, but if he doesn't, you might be covered under your homeowner's policy if the helper is injured.
To finally get to your question as to whether the waiver would be legal: Probably not. There is no way it would be legal for the helper. Since a waiver must be knowing and completely voluntary, a third party usually can't waive for another. Beyond this, there is a very strong public policy in California that there must always be some form of compensation for workers who are injured. Without doing research, my guess is that the courts would strike the waiver as against public policy.
Your best insurance to avoid problems is to hire a licensed contractor who has worker's compensation insurance. If a person isn't licensed or doesn't have worker's compensation insurance, it is a huge red flag that they don't have the skills to do the work you've hired them for. In my experience, unlicensed contractors cause way, way more problems in proportion to licensed contractors.
