。
She's skin and bones.
Literal
She [topic-は] skin-and-bone-skinny [adverbial-に] is-thin.
がりがり is a wonderfully versatile mimetic. The sound sense is the crunch of biting something hard, or rough scraping; the state sense covers being uncomfortably skinny — like a body so bony it almost has a 'crunchy' edge to it, the same word that anchors がり勉 ('study grind, intense studying'), the cultural archetype of someone studying so hard they wear themselves down. ~に痩せている ('is thin in [manner] way') uses the adverbial に to attach the mimetic to the resultative verb 痩せる ('lose weight, become thin'). The whole package paints someone alarmingly underweight rather than just slim.