How to deal with living in a shared house

[nzborn]

My Delima, I live with friends a married couple and his son 23 ish from his first marriage,his mum has shifted in 73 ish who still works part time I share a downstairs toilet with the Grandma and Grandson mainly Grandma leaving a mess in the toilet bowel and makes no attempts to clean it.
I've tried leaving it but then you can observe different days worth.I've spoken to my friend about her Mother in Law and they have no Idea apart from talking to her which no one wants to do.
Please help nb we can't put up a sign in the toilet.

Ew.

I would say something directly to her if you're CERTAIN it's her.

I would say "The toilet needed cleaning after you'd used it so I'd really appreciate it if that's something you can do regularly thanks."

What's she going to say to that??