Should I report my housemate for smoking weed? Watch
Student life, in partnership with UEA
If you report her to your landlord, there's actually very little they can do. They won't be able to gather evidence to prove anything, and evicting someone other than via a Section 21 or for non-payment of rent is quite tricky (and even the non-payment route isn't easy). If the landlord somehow manages to evict her, chances are you are all jointly and severally liable for the rent- so everyone else will have to cover her share of the rent. Equally, if you've got a shared deposit, it's very likely the landlord will deduct money from this, which again affects everyone, due to smoking during the tenancy. Maybe, if you've got an especially good/interested landlord, they will have a word with her, and that might scare her into stopping, but it could just piss her off and make her into a worse housemate.
The police won't be interested if it's just for her personal use, and that's a very extreme step. Even if the police cautioned her for possession, she'll still be living with you for the rest of the year, and if you've already called the police on her, she has no incentive at all to be a decent human being. Like, smoking weed indoors isn't great, but she could be a lot worse. Worst case scenario, she ****s off, potentially leaving the rest of you having to find her share of the rent. Also, this is a pretty extreme thing to do to someone, so it could lead to other housemates turning against you.
You could report her to the university, but they don't really have any powers to do anything- this isn't the sort of offense people get kicked out of uni for. If the uni are your landlords, see the first option. They might be more interested in getting her to stop, but you've still got all the major downsides and the uni still has to follow the law.
It's not ideal, but things could be worse. You've got, what? Three more months of living together. Don't live with her next year. Move out as soon as you can. Try to impress on her that smoking indoors will reduce everyone's chances of getting their full deposit back- if she's otherwise decent she might stump up for professional cleaning or something.
If you don't want to then you do your own thing but leave other people to enjoy their lives!