What concern does Sybyl share with Mayra, and how does Mayra respond as noted in the memoir, Wasted: A Memoir of Anorexia and Bulimia?

Marya and Sibyl were friends and roommates during their time at the university newspaper. Sibyl tells Marya her habit of checking out her rear end each day in the apartment's full length mirror is ridiculous. Later at her parents' Christmas party, Sibyl shows concern about how thin Marya has become, and although Sibyl insists that Marya is sick, Marya tells her friend she is fine.