I've (29m) been with my girlfriend A (27f) for 18 months, we just moved in together about 6 months ago, we both love plants.

She brought a dog (GSD) with her (I've never had pets) but her dog continues to shit in the plants instead of the litterbox. She is insistent it's not a big deal but I hate it, it smells awful. I've finally broke and started throwing away the plants her dog shits in. This sucked at first because I was throwing away these massive floor potted plants and then we were left with smaller/medium sized plants.

This is the issue, I don't have any proof but I think my girlfriend is the one shitting in the plants because the only plants we have left are in a glass cabinet and the doors are always shut unless they are being watered and how could a dog even shit in that small of a plant????????

What do I do?

Edit: to clarify the litterbox thing, i should have been more specific, it's a fake patch of grass you order for dogs, not an actual cat litterbox. We call it a litter box because it's an in a cardboard box for easy set up/disposal.