Having a bong is not illegal, and having a bong at your place doesn't in itself mean that you smoke drugs or have a drug problem. If i had a group of friends and one of them lived in a caravan or bungalow, and the rest slept in bedrooms in the main house, i know where i would be storing my bong (if i had one).

I'm not trying to minimise the issue, and it sounds like there are other things going on as well, but try not to think too far ahead before you work out if it is even her bong in the first place and if it is being used. Maybe she is doing ok, and a mate left it there.