Why are you not friends? Has there been a falling out or just a natural growing apart? If you’re not even friends then I would question why you even need to find out or ask. It’s a big thing to trust someone with, so before you raise it with her ask yourself how much you trust her and how you will feel if she shares it with the ex? Do you trust him? They might be a lot more ‘out’ than you are and may be much happier to share this with others than you are so don’t assume just because they share your secret they’ll share your desire for privacy - the fact they leave diapers and packs etc lying around suggests they don’t care if other people know. Do you feel the same or do you only feel comfortable sharing with her because you’ve found out she’s into it too? Will you be okay with it if they tell anyone outside of the house?
Obviously lots of people here want you to confront because living with an ABDL girl is the ultimate fantasy, but remember real life doesn’t usually work out like a story. What would you hope to gain by telling her and is there anything for you to lose? If the risks (Eg: being exposed to others) outweigh the possible benefits it’s not worth it.