No. The contract has been fulfilled. This medical condition should have been discussed and put forth in the contract.
However, you may still be left with a moral conundrum. Will you feel bad about this situation?
The only ethical implications you may face from society would be "did you do your due diligence when coming up with the terms of the contract?" Did you, as a proprietor, establish all required facts/data that were needed to execute your end of the contract? (probing questions) As an example, if the customer ordered a cake, you would probably ask, "what kind of cake? Chocolate? What kind of frosting? (Allergies?) etc. Yet I sincerely doubt that you could be completely held responsible and at fault, because after all- you did deliver what was asked for. It (to me) seems like it should be the customer's responsibility to let you know of any medical conditions that might hinder.
From the perspective of 'big food', take for example General Mills, it is an ethical (and legal) obligation for them to properly label potential hazards. This is because there is no way for me as a customer to ask specifically and directly to GM, "Does your product contain peanuts?"
Hope this helps...