Hello, Some good advice above,
I recently went on an IICRC that covered mold, I would say the carpet should be thrown, the building needs drying, looks like a serious lack of ventilation.
Yes it can pose a health risk, its scares me that a family could move in here unaware, if they have ashma or allergies its really bad for them!
Really an expert is needed to advise. Tell them to get chemdry in because they know all about this and have the equipment and no how to deal with it.
looks to me as though it needs cleaning, drying and fogging with a antimicrobal, ppe should be worn. conditions need to be looked at so it does not revert back to this state.
PS. If it were me I would walk away and tell them they need an expert, and suggest they call chemdry or simular flood damage company.