Gretel watched that expression of his carefully, and for a good long time; she could see that he was being honest; he wasn't jumping into something just to placate her- that was important. Even if the potion ended up helping him, if he did it before he was actually ready, it would weigh on her.
"Come on then," she finally said, scooping up the Grimoire in one hand and lacing the fingers of her other in his in order to head to the little room off to the side- the one she absolutely refused to call a 'lair'. "And at any time you decide otherwise, just tell me." Up until the moment he drank the tonic, she gave him an easy out.
She went through the ingredients list quickly- first adding another piece of wood to the iron stove to built heat for a pot of holy water (from Sam's flask), then everything else in a small wooden bowl on the work table: a sprinkle of alter ash, a few dried herbs, a drop of honey, and three drops of 'honest man's tears'- those she'd collected from Chirrut.
"Get that strip of cloth ready for me-" she asked him, pointing with the tip of her curved dagger toward a pile of folded 'bandages' made from destroyed materials of 'donated' clothes. While Sam turned for the task, she quickly drew the blade over the inside of her arm, her breath held against the sharp sting. She pressed a couple fingertips against her skin, letting the blood stream slowly into the bowl while her lips moved in words that had no sound or English meaning. As she whispered, a light- faint at first, then growing in brightness as she continued- seeped through the red in the blood in the bowl, turning it gold.