Cutting the roots of a palm can defintiely do some damage. If its a mature, established palm, it will live, but will still be a little bit set back (probably wouldnt put out a lot of growth this summer or at least not as much as usual). As for cutting the trunk, technically they can survive that, but a wound to the trunk can really weaken it and kill it in the long run (and if he does get to the center of the plant it will be killed.
If this is something that has to be done to save your retaining wall then give it a try! Washingtonias are fast growing and cheap in your area so if he kills this one then you can get another one and maybe plant it in a better spot too. I dont think he will kill the palm, but for the record, the palm will definitely not be that happy!

Welcome to the fourm btw!
Good luck!
-Alex

Thanks for the information! I've been lurking through the forums for quite some time, as I have 26 palms in 5 varieties at my house. The information here is good. I always seem to find the answer to a question I may have, except in this case.

Can you get a second opinion from another landscaper in your area? You might even call an aborist or your local nursery. When I was looking to have a deciduous tree on my property trimmed, I was amazed how much I learned from the aborist.

Like Alex said, I don't believe cutting the roots will kill the plant although it might slow the growth for this summer. I'm still unsure about the trunk trimming...just seems unnatural. If anything the tree might become susceptible to some disease or fungus that way. Depending on how the landscaper cuts it, it seems it could make the tree weaker and susceptible to eventual collapse due to weight.

If you can post a picture, that would help too. Good luck and keep us informed of your decision. It's how we all learn. :)

As long as one cuts the palm on one corner/side only, I doubt this will kill the palm, and it may not even set it back any. These are pretty tolerant of root abuse. A large establish palm is hard to kill that way.

In my opinion cutting the roots won't hurt the palm but the roots will grow back within 2 year and so it won't really fix the problem.
cutting into the trunk is another story. That could leave it susceptible to stem rot and other problems.
Arthur the date palm guy

Well, it's done. I decided to take my chances because the tree guy told me if it dies in a year, he'll remove it free. He took a lot of roots out below ground level, as well as what appeared to be roots above ground level, too. Above ground level, about 3" are gone all the way around the trunk, up about 1 foot. Unless there is something I need to do in terms of tree first aid, I'll post again in a few months with a palm health update. I'd post a picture of the tree, but I'm not quite sure how to do it.