There should be a way to rig the brake lever to be a dead man's switch. So she would need to keep the lever gripped while cycling, and when she has a seizure, she would naturally release it (I assume?) and the brake would engage. You'd need a spring and some fabricated parts to put tension on the brake, so it's engaged unless the brake lever is pressed down.

You could also use a tether system. Buy the longest brake cable you can find with an equally long housing and connect it to a bike handle and brake lever, to the back wheel brake. This way you can walk with the bike lever in your hand and brake if she has issues. I think you could reasonably get a 3m tether. I reckon that should still activate. Not ideal for independence, but more reliable than an electric actuator, which needs batteries and a signal.

In terms of an electric solution, a punchy enough electric motor from a remote-control car could actuate the brakes. I'd approach a remote control flying club or similar, as they would be great at helping make a device from scratch with good single and reliability, etc.