Hi,I'm Miranda, and I'll be happy to help you with your question.
Generally, oil-based stains are not removable with water. It usually takes mineral spirits, turpentine, or paint thinners to remove those products, which would probably kill the plants anyway. Therefore, rinsing the affected plants off with water probably won't work.
My suggestion would be to try pruning the plants back as best as you can. Don't prune them too far, but enough to at least remove some of the stain. This is probably your plants' best chance for survival. However, unfortunately, there is no guarantee that it will work. Your plants will probably stand a better chance if they are older perennials that have been in the ground for a few years because the roots will be more durable.
As for the contractor, I would say that they are definitely liable if the plants die, unless there was something specifically stated in a contract that they weren't responsible for such things. Take plenty of photos of the damaged plants, especially if they die, and then call your contractor and complain. A reputable contractor will most likely apologize and be willing to give you some sort of discount on their service or replace your plants entirely.
I hope that answered your question. If you have further questions, please ask. It's been my pleasure helping you. Please remember to rate my service positively, as rating me is the only way I receive credit for my work.