Girlfriend insists on using natural soap (dr bronner’s liquid), making the sink slow.

On a septic system. She suggested it was from me shaving (doubtful, I don’t shave often), so I plunged the sink. It was better for about a day or two then got slow again. Pulled the stopper, and there’s white fatty hunk built up in the pipe. Used a zip snake to make sure there weren’t any clogs in the trap. I’m pretty sure it’s built up from the liquid hand soap she insisted we change to. What’s a good way to keep this from fucking up any more pipes all the way down to the septic tank?