Probably not a problem. Most people with seizure disorders or epilepsy lead relatively normal lives with a few accommodations.
I would think that a discussion with the person or their family might be in order to ascertain exactly the type of seizure, the frequency, duration, etc. Seizures are rarely life or health threatening intrinisically, unless the seizure lasts beyond 5-10 minutes without let up - then it's time to call 911 to fend off respitory arrest and other complications. Until then, lead the person to a place of comfort and rest, loosen their gi or obi and let the seizure run it's course.
Avoid some of the old remedies that do little good and can cause harm.
Also, determine what anticonvulsant medication they are taking and if there are any neuromotor or concentration complications. Most of the newer generation of meds are fairly discreet, but every now and then you will find someone taking a dinasaur drug like phenobarb.
There are a wide variety of seizures from absence types where people simply blink off for a few seconds to those who have full classic grand mal types(now called clonic tonic I think)
Some seizures develop a pre aura that alerts the person the seizure is happening, others don't.
If the person has nocturnal seizures or very rare day time seizures, but has been seizure free for some time, it would seem within reason that karate is appropriate. If you want to be sure, you could ask for a letter of release from their neurologist to be safe I suppose.
The main concern would be that in sparring the person could be vulnerable if a seizure took place at that instant. Also, falling to the floor could create an injury. These would be improbably, but possible.
However, stress can be a precursor to a seizure, so sparring might be entertained with caution.
Get informed and accommodate accordingly.
[This message has been edited by JohnC (edited 07-19-99).]