Yes, diacetyl can be unhealthy, but only in LARGE quantity exposures, such at a place where is is manufactured or where its put into the popcorn bags. That can be addressed by proper ventilation systems.
At the consumer level, making a few bags a day in your microwave & eating the popcorn is not a harmful exposure.
wondering if the guy sniffed the steam coming out the 2 bags a day for a head rush.