A female friend of mine had a fight with her roommate over some trivial matter. How should I comfort her?

The roommate who can’t be slapped in a quarrel must have something wrong with her. First, from the perspective of your girlfriend, tell her what’s wrong with the roommate you’re quarreling with, and then try to persuade her. It is a kind of fate to meet each other in the vast sea of ??people, let alone living under the same roof. If you have similar interests, say more. If you have disagreements, say less or not at all. There is no need to cause disputes, make everyone nervous, and make the meeting awkward. If you have a quarrel with good friends, you will definitely regret it later when you calm down. If you are not a friend, then the best way to eliminate your enemy is to become a friend. Once she understands these truths, she will naturally not be angry anymore.