This is a conditional question.
This employee would be protected under the ADA. If reasonable accomodations can be met (such as snack breaks), and the employee is able to safely carry out the job with these accomodations - they will be protected. A few years ago, UPS underwent a lawsuit regarding this - it involved 2 employees. One who was able to still safely do his/her job with snack breaks - deemed protected (UPS rehired and made compensation for loss), the other employee was unable to safely do the job with snack breaks (documented safety concerns, employee was not rehired). The accomodation agreement was made in writing and signed by the employees. Protip - Always have things in writing.
That's only the ADA side of things. If there is a state or city regulation regarding certification (such as the earlier posting regarding CA ambulance permitting - if valid), then that is a seperate issue. If the job requires a permit or license that the employee is unable to obtain or maintain due to the condition, they are unemployable on the basis that they do not have the permit or license (not because of the reasons why).
As an employer, I'm protected from this with the simple question on the application similiar to Kat's regarding any condition that may affect them safely doing the job. If they have a condition requiring reasonable accomodation, it is the employee's responsibility to disclose such information - since much of this information is confidential unless they personally disclose it to me. If they do not, and something occurs, they have "lied" on their application (unless it's a sudden onset or unknown condition). Once they disclose it to me, then it's my responsibility as an employer to investigate reasonable accomodations to allow them to safely do the job (normally a simple meeting with the employee on how they handle their condition, anything I can do to assist, etc). If they do not comply with the accomodation (such as not taking snack breaks when I've made them available), then it's back on them.
(I'm not a lawyer, just have many years in HR)