The first error is, actually, that you're missing a designator of whose roommates they are:
"She looked enviously at roommates, peacefully asleep with their stomachs full." should be "She looked enviously at her roommates, peacefully asleep with their stomachs full."
To be very pedantic, yes, that should also be "roommates, who were peacefully asleep with their stomachs full," but I would rarely blink an eye at a construction that omitted "who were," leaving it as understood. (But then, I often edit things down to have a lower word-count, which means leaving out words that will be understood as "invisibly present" to most readers.)
If, however, the words after the comma were ambiguous, able to pertain to both subject and object, you'd need to define it. "She slept restlessly beside her roommate, sleeping with her stomach full" is a slightly awkward phrase in general, but especially because the part after the comma could refer to either the "she" of the sentence, or "her roommate" -- and would probably attach to the "she" in many people's minds. For that, you'd definitely need "She slept restlessly beside her roommate, who was sleeping with her stomach full."