Tristan was fast to react once he realized what was going on and as soon as Charlotte sank to the ground he was there to partially catch her. If he couldn't stop her from falling, he could at least help make the impact a little easier on her. Arms encircled around her and he gently helped her to the ground, and without even thinking about it he drew her closer to his chest to hug her.
She needed comfort, she was in a completely foreign environment that was being somewhat forced on her because of the woman in her head. Tristan could only imagine how suffocating that must feel. And completely overwhelming. He honestly didn't think he'd mind if Fairyland appeared out of thin air but this wasn't about him, this was about Charlotte. And Charlotte was having a hard time dealing so he was determined to help her.
"It's okay, Charlotte," he murmured into her hair, resisting the urge to breathe it in pretty well once he reminded himself how completely inappropriate this was. The way he thought of her... It didn't matter how he tried to justify it, it was wrong, and he shouldn't be thinking about that right now. He should be focusing on making sure she was okay, so he did just that. He'd keep his arms where they were around her until she didn't want him to anymore. "You're okay. I'm here for you."