He wanted it, needed it and he'd asked her, he didn't like it but truth was he didn't have to like it, not even for a minute, he just needed to understand how important it was, and Ruby was pretty sure he understood so she did what she had to do and cut into her flesh with the sharp but not magical knife.
It didn't hurt, sliding the knife with worrying precision and accuracy across her arm, clipping the vein and letting blood flow freely from it. She knew how deep to run the blade, so why would it hurt, she'd done worse and had worse done. Besides as long as she was in Coma Girl it replenished itself. It was most definitely useful for the purposes of this. No accidently dead meatsuit.
The cut made Ruby's eyes found Sam's again. He was watching her, and she could almost see the part of him that wanted it more than anything else at this second, bubbling under the surface, all Sam had to do was listen to that part of himself, and Ruby knew that he would do it.
"Go ahead" she told him calmly, and waited leaning against the tree and holding out her arm at an angle so he could lean back near her and hold it at the elbow comfortably, she found herself relaxing as she waited. Waiting for the sensation that would tell her she'd helped him finally.