I would seek legal advice either from your business lawyer about a proper waiver of liability to be signed by the customer.
This reminds me of the waiver your sign at most tire shops when you have one tire replaced instead of replacing both on that end be it front or rear.
It basically states that you have informed the customer of the manufactures' safety recommendation about the risks associated with changing only one tire and they hereby release all liability to the above company and technicians for any property damage, loss, or bodily harm that might occur.
Again, proper legal counsel will be able to properly word the language for use in court.
Hope this helps.