I think it's important to note that it's the performer's responsibility to keep themselves safe, and taking you into their confidence is good judgement on their part, but you are not responsible for anything other than accomodating to the best of your ability and keeping them informed. If you are concerned about liability, keep a diary that will show due diligence. You can ask to disclose information to a company manager.
If a performer cannot be around electromagnetic devices, I would assume that means they have a pacemaker. (If not, it would be good to have more specific info.) Then you can say, "In this venue, there is a microwave in the green room and there are 4 solenoid releases onstage, 30' over your head." (That's just an example - most solenoids may be perfectly safe, and certainly would be at that distance.) Asking the TD and/or crew chief about equipment is a great idea. You don't even have to disclose: "How is the balloon release triggered? Is that a magnetic device? Where is it located?" Take a tour of the venue. In ours, the dimmer rooms are very far away from the stage, but perhaps they are closer in yours. After you have informed the performer where the devices are (and ensuring they are not relocated without your knowledge), it is up to them to maintain their safety.
If a performer is prone to seizures, then having an ASM backstage who knows how to handle a seizure is ideal. Again, the performer can tell you how best to handle a seizure, but it will likely mean just sitting with them until it has passed, keeping them from injuring themselves on nearby objects, and assuring bystanders that all is well. Please don't put anything in their mouth!
I have had performers tell me they had heart issues, epilepsy, panic attacks, were undergoing chemo, and one asked not to be scheduled at certain times so they could attend N/A meetings. I have worked with wardrobe to accomodate a stomach feeding tube and a dialysis pump. I have worked with a diabetic who was still learning to control his levels, and we kept a sandwich nearby to put into his hand if he started to slow down or get brain fog.
The Red Cross is a great resource for advice on how to handle any issues. I bet if you called your local chapter for advice, they would be happy to offer more qualified advice than mine. Whenever I recertify for CPR & First Aid, I come with a list of questions just like these!
The fact that you have been taken into their confidence is wonderful! And, you probably don't have to tiptoe around anything. If you can find a private place to talk, I bet they will give you more detail about how they manage their particular issue, once they know they are in a judgement-free and confidential zone. If it's something they don't want others to know about, they will feel relieved to have a confidant.
With permission, make a list of pertinent info: medical condition, medications, contact info for Dr, contact info for their partner or parent, etc, that you can keep in a sealed envelope (I keep it in their valuables bag, so it's with their wallet). If necessary, you have the envelope and their ID handy to give it to EMTs to ensure the quickest and best treatment in an emergency. If you don't need it, you hand it back to the performer to give to their next SM or shred it. (I ask everyone to fill out an in-case-of-emergency form, and everyone has sealed envelopes with either volunteered information or blank paper inside, so it's not unusual to have a sealed envelope in your valuables bag.)
The gentleman who I worked with who had heart issues did indeed have an episode, and after his Dr. was contacted, he was fast-tracked through the ER while his Dr. was en-route to the hospital. Information is the stage manager's superpower. A reputation for discretion gets you more information.
Lastly, break the confidence if you feel it's a matter of life & death. Most company managers understand discretion as well. They deal with as much of this stuff as you do.
-Colleen