MadEury, I think a little more info might help. Are the bathrooms adjacent to one another or is one upstairs while another is downstairs? Did her tub have any problems draining? Did any other fixtures clog up or back up? Do you hear any sounds in the pipes when other fixtures are being used?
My first guess is that its possible there is a clog somewhere after the drain for both tubs. It's also possible that the wrong fittings may have been used for one or both of the tubs, allowing water to flow from one to the other. I wonder if they are mutually wet-vented....
I'm not an expert. Maybe one of the real plumbers here can help.