“Hey I know this is awkward, but I’m noticing you’ve been using a lot of tp and just wanted to make sure everything is okay.”
That opens up a conversation without phrasing it like a confrontation. In the meantime I’d just only pay for your own toilet paper. Keep it in your room if needed.
There are some good comments about confronting your room mate about it, for the plumbing though, I would recommend you take a picture of it everytime the bowl is filled with toilet paper so if anything were to happen, you can tell your landlord exactly whats happening and that it isn’t your fault
I was in a similar situation with a roommate back in college. She never bought toilet paper and go through A LOT. After bringing it up a couple of times and nothing changed, I started keeping the toilet paper in my closet and would bring a roll in with me. Super petty, but oh well.
She ended up being a terrible roommate for many other reasons