It was the smell of old age, and dust, and unwashed clothes and molded stale food, and it intensified the closer she got and smelled of decay, too. Fred didn't begrudge Roger the need to cover his bloody nose.
Quite a few things happened in a quick succession, then. The hunched old woman walked into the light of a window, and Fred got a good look at her - her hands seemed blue and mottled and her eyes were thick with cataracts and sunken into folds of nearly translucent folds of her face, skin dotted and broken with liver spots and broken veins. He thought she was blind, maybe, but she shuffled right to them which made it less likely.
And then before he could do anything - like hold his own nose, maybe - the woman had lunged at Roger, and really lunged, not as if she was scared or reacting, but as if she was going for his bloody throat.
"Oi!" He called, and no matter who she was, or how old and sad, he grabbed for her, arms around her waist to pull her back from Roger. Her skin was cold, freezing even through the heavy robes, and as he grabbed her something else happened... the skin seemed to slough off under his hands, and he let go, alarmed, just in time to see the body collapsing, and a great snake pouring from a now gaping hole where her neck had been.
He raised his wand, but a bite to his forearm - so fast that he hadn't seen the snake sodding move - sent his wand spinning up toward the ceiling, then a powerful blow to his stomach from the tail knocked the breath out of him, and Fred fell backwards onto a moldy sofa, into a mound of filthy clothing.