In the US, it is the end-user's legal responsibility to have safe machines. You could legally sell a robot with no guarding or safety of any kind to an end-user and have zero legal liability for it, because the end-user is responsible for knowing the safety requirements and abiding by them.
That said, we also live in a very litigious time and place where even if you aren't legally liable, you can still be sued. Even if you're in the right and can prove it, it still costs time and money to defend yourself.
Personally, I would politely decline the job, stating the unsafe design of the machine makes it too much of a liability for you. Even if the customer offers to sign a waiver, there's the fact that they aren't the only person who can sue you. If someone is killed or maimed due to the unsafe machine, the family of that person can sue me personally. Any waivers the customer signs don't apply to the families of their employees, so ultimately there's still a risk, there.
That said, we also live in a very litigious time and place where even if you aren't legally liable, you can still be sued. Even if you're in the right and can prove it, it still costs time and money to defend yourself.
Personally, I would politely decline the job, stating the unsafe design of the machine makes it too much of a liability for you. Even if the customer offers to sign a waiver, there's the fact that they aren't the only person who can sue you. If someone is killed or maimed due to the unsafe machine, the family of that person can sue me personally. Any waivers the customer signs don't apply to the families of their employees, so ultimately there's still a risk, there.