Was she wrong for telling the landlord that her roommate is running a business out of the apartment?

If you have roommates, it’s very important to have a frank conversation about your guest policy up front. If you don’t, it can lead to months of resentment and bitterness, making the living situation miserable. 

Some people are totally okay with their roommate’s boyfriend or girlfriend staying over every night, using the shower, and eating the food in the fridge. A lot of people are not okay with that, but they feel uncomfortable verbalizing that discomfort to their roommate, because they want to keep the peace. If they hold their discomfort in long enough, it becomes resentment, which is bad for everyone involved. You should ask your roommate if they’re okay with you having guests over, but it’s also on them to let you know when they’re not cool with the number or frequency of guests. 

If your roommate is constantly having their partner over, that’s more tolerable than having a huge group of friends coming over at all hours to be loud and obnoxious. The roommate in this story is hosting a study group from her apartment, and worse than that, she’s paying her classmates for the pleasure.