Reol'ran was frozen and almost destroyed in both body and soul by the river's oppressive might as it began to crush her, but some sort of remnant power in the Mirror Shard surrounded her. She instinctively squeezed it all out, and the River ended up freezing her body entirely rather than murdering her straight out.
She survived.
And since it was in her own Destiny to be in the Hunter World when Mark appeared there, her icy form floated in the river and in a wild and coincidental wormhole incident, got teleported through space to appear in a random, abandoned Laboratory.
All Heaven Soul Race members had some unique perks. One of the perk was that they had a natural dungeon around them that would keep away the burning influence of the World Wills of different worlds, so, the Hunter World's WIll could do nothing to her.