I'm sorry about what happened to your friend! Did she have her horse on an HYPP-based diet, keep the Kayro syrup on hand etc? Granted the Kayro syrup is for emergencies when an attack happens, and if you are riding when an attack DOES happen...then it's hard to get the Kayro in right away. But if you follow strict guidelines with an N/H horse, it CAN lessen the chances of an attack.
Personally, I would not own an N/H horse for the mere reason that I'm not an Impressive fan to begin with. It's not a bloodline I look for in my performance horses which has nothing to do with the HYPPP issue.
Impressive isn't my cup of tea either, HYPP aside. If it was, I'd only have N/N horses. I have several Poco Bueno bred horses, I'd not own one of that line if it was a HERDA carrier or affected. For me it's a matter of why take a chance when there are plenty of other nice horses out there that don't have baggage.