I have sent this question to my friends who are professional disability advocates, and I hipe to be able to provide you soon with the legal answer.
However, it seems appropriate to give you some guidance now.  Having the woman with epilepsy carry the dog was the wrong position to take and of that I am certain.
But it is important to note that service animals are used for a myriad of disabilities , and all are considered appropriate accommodations under the ADA.
Service dogs are used by the blind.  There are service dogs that act as disability sensors.  But the least understood and most oft discriminated against are service animals for those with mental health issues.
Perhaps Terri Nolinske on this list could be contacted to provide ADA training.

Yearous, Jenny Dee wrote: 
>  I am posting this for a colleague you can e-mail her 
> directly if you like at the address listed below. 
>    
>  Would you please ask the 
> museum list serve if they have any guidelines for handling assistance animals in 
> their museum?  
>    
>  Are there any requirements 
> for them to have registration or some kind of identifying leash or some other 
> adornment? 
>    
>  Recently we had a visitor 
> in with a dog that sensed the onset of her seizures.  The dog had a vest 
> that clearly identified it as an assistance animal.  Unfortunately our 
> staff told her that the dog needed to be carried (it was a rather 
> hefty Jack Russell terrier) and I don't think that was appropriate.  
> Today we had another woman enter with a dog under her arm and staff jumped on it 
> right away since they assumed since she was carrying it that it was not 
> necessary.  
>    
>  Beth Campbell   [log in to unmask]   
>  Visitor Services 
> Coordinator 
>  State Historical Society of 
> North Dakota   
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If you decide to leave Museum-L, please send a one line e-mail message to [log in to unmask] . The body of the message should read "Signoff Museum-L" (without the quotes).