Tile experts, please help
My sister's house is being restored after a fire. There have been a number of issues with quality of work that was done. She just moved into the house six weeks ago and there are already black lines appearing in the grout joints in her master bath shower. Similar black lines are visible the grout in another bathroom that has only been used 4-5 times. The exhaust fans have been used following every shower. She sent the contractor photos of the tiles and black along the grout lines and he responded: "I can't keep mold and mildew from growing on the grout. Bathrooms need to be aired out (exhaust fan) and kept clean and dry."
Do you think the black lines are mold? Are they caused by poor installation? Or are they caused by use of the bathrooms? How can this problem be solved?
Attached is a photo of the tile & grout lines in the hall bathroom.