On the contrary, Marian thought him asking her to come was the only sensible thing he'd done since it had all started. She was genuinely worried for him. Yes, she supposed his ghost could, at some point, hurt her. Randomly flying objects and all. And hurting her would hurt Robin emotionally, so she knew that, despite the ghost's quarrel not being with her, her safety wasn't guaranteed.
But really, when was the last time she could swear she was safe? Her mother disapproved of how foolishly she took her life for granted, but she had also readily accepted her daughter's spirit. It made Marian wonder what her father might have been like had her mother lived. Sir Edward had come to accept Marian's double-life, but it had taken time and threats of losing her. Perhaps if the Lady Katherine had lived, he'd have been more open and more willing. Or maybe that wouldn't have been the person Marian grew up as, being sheltered by her mother instead of witnessing too much of her father's work.
All Marian knew at that moment was that she wasn't going to allow Robin to suffer alone. He wouldn't ask it of her if he wasn't worried. Maybe even afraid, though he'd never say it. And while he was worried for her safety, she wouldn't allow that to get in her way. This was her husband, after all, even if not in his time. She'd vowed to stay by his side and she'd meant it.
Marian arrived at the flat with a plastic container of rock salt and one of the iron pokers she'd found downstairs in her arm. Just because she was bravely defending her husband didn't mean she was going in unprepared. She thought of knocking but reconsidered. He might panic and tell her to stay away. And she'd have none of that. Instead, she attempted to twist the knob, finding it gave way easily in her hand. Lightly, she pushed the door open and looked around, despite knowing if the man was present, she wouldn't see him. What she did see was the love of her life, looking exhausted and beaten and more than a bit wary of whoever was in the room. "Oh, Robin..." she said softly, simply standing and watching him for a moment.