It needs to be managed VERY closely> They need to keep an inhaler with them at all times. There are many Meds that can Help. I lost my father severail months ago to COPD. Yes it is dangerous and it can become worse at times. I do not wish to scare you. But anyone with COPD needs to be watched espacaily at Night. A suggestion. Have you're sister have a Baraim Swollow Test. They have found during sleep COPD pataints have issues with the breathing/ Swallow durining sleep and it can cause attacks to breathing. A sleep study also.