Ideally, Ruth will make a big production right at the start of the meeting that Mary told her there is a non-student living in the dorms against the rules, and that she (Ruth) will find out who it is. Ruth gets to maintain her 'Ruthless' reputation by making it sound like horrible things will happen to Becky, and the blame for her finding out about it will fall straight on top of Mary's head. Becky is apparently well liked by the other girls in the hall, and they've been watching out for her, those that know she's living there. So I'd be willing to bet there would be more than a few angry people looking to get some payback, or at least treat Mary with a fair bit of hostility. More than she already gets, at any rate.
This would also pull the teeth on half the threat she's holding over Ruth, that Becky was living there and Ruth isn't doing anything about it... But publicly stating now that Ruth does know about it, she'll deal with it in some non-specified by likely unpleasant way. Billie is clearing out of Ruth's room, and the booze has been removed, so even if Mary goes to the faculty about the two of them, it will pretty much come down to her word vs Ruth's. And Ruth has said that staff loves her because she runs a tight ship. She can also point out that Mary is a trouble maker, and suggest she's just trying to get back at Ruth after being caught using transphobic slurs to another student... Basically, unless Mary can provide evidence that Ruth was doing wrong, going to the staff is going to go bad for her, not for Ruth.
