I hate to be alarmist, but yes choke is life threatening.
With all that stuff in her airway she can't breathe properly and there is the potential that there is something large enough (or enough small stuff) to case a complete blockage.
Also she's not eating or drinking which means she's dehydrated among other things which puts her at risk for secondary problems. I'd also be concerned about her getting shocky at this point.
Call another vet (I always had a list of 3, but I guess I was lucky enough to live in an area where that was possible), or at the very least get your vet out there as soon as s/he and make sure s/he understands that you feel this is an emergency and your horse is still actively choking.
Sorry I don't have any suggestions for you; I've, thankfully, never had a horse choke that badly. I hope you can get a vet out to help you and she's feeling better soon!