I am not a doctor, but remember that Liam Neeson’s wife, Natasha Richardson, died after bumping her head while skiing. She seemed fine, until she wasn’t. The couldn’t get her to a hospital that could treat her in time, Quebec at the time (2009), having no medical helicopters.
I would hope that the hospital that Dagny went to throughly checked her out; if she died from the assault in the bathroom the girls who attacked her are going to have some legal problems. It is being speculated that she died from something other than the beating.
So tragic. She didn't help matters when she waved off Liam's desire to put her in the ambulance that had arrived on the spot, he wanted to pack her up straight to NY, where she eventually died at Lenox Hill. It would have been a ballbuster to get to that clot but had she climbed in that ambulance that came to Mont Tremblant, yeah, she might have made it.