I'd agree with Frank. You might build them from some published plans to have an accurate height, might even have your friend sign off on them. I don't view them as anymore of a liability than if you built them a step stool or bed.
Also, If your truly concerned, you can always setup an LLC and do the work under that business entity, which I'd recommend to anyone building products for sale.
To a small child, the perfect granddad is unafraid of big dogs and fierce storms but absolutely terrified of the word “boo.” – Robert Brault