Since the app is intended to be used while sleeping, it would be pretty easy to include a preprogrammed address in the 911 message (perhaps with a geolocation 'sanity check' just in case the wearer is traveling).
I think notifying a friend or family member is probably the most important thing. They can follow up from there.
Text-to-911 is growing in availability (in the US, anyway), but it's far from universal: