A normal repair and restoration of a marble fireplace like this would involve removing the fireplace from site. Separating the individual pieces, before removing the old adhesive and cleaning them up. I would then re-assemble or box up the two sides, which would be nice and square to make fitting easier when returned to the property.
HOW TO RESTORE MARBLE VANITY TOPS
How do you restore a marble vanity top, or how to polish a bathroom countertop? When the marble is relatively new and has little signs of use and no dull spots. A polish like Lithofin MN polish is one of the best to use as it is easy to apply and buff to a shine.