Given that your sister dorms with 6 other people, I'd say the best bet is to find a better place - one where she doesn't have to cram and live under a food-taking "dorm master," imo. That way, she and that friend could use their money for rent and food and transportation for better use.
If that can't happen, is there any public storage they can access nearby? If so, they could store the things they don't need at the moment to free up space.
So you say she has a microwave. Here's my tip for that:
This is the first time I'm hearing a yonana and I swear it's the sole answer to my finding-a-way-to-eat-ice cream-and-healthy-at-the-same-time conundrum.