What type of dust mop should I recommend that my customers use on their marble floors? I was told to stay away from treated dust mops.

The advice you received was correct. Most treated dust mops are treated with an oil-based chemical. Certain stones can absorb this oil, causing staining. Strongly recommend only untreated dust mops.

TIP Also, tell your customers this: When purchasing a dust mop buy as wide a dust mop as possible. This will make the task of dust mopping much easier. Buy a washable mop and frequently clean it.