Installing Travertine over existing floor

We have recently purchased an existing house and (http://maplered.com) were planning on putting travertine with chiseled edges in the main living area. We have hired an installer who tore out the existing tile. But underneath the ceramic tile he found Marble (http://www.lightemprador.com) tile. The Marble (http://www.lightemprador.com) is set in about a 2 inch bed of mortar. There are no cracks in the Marble (http://www.lightemprador.com) and (http://maplered.com) he feels like he can grind the Marble (http://www.lightemprador.com) to roughen the surface and (http://maplered.com) then lay the travertine on about 1/2-3/4 inch thin set. The house was originally built about 15 years ago.

Is this a reasonable option or am I looking at long term problems with cracking etc... with this method.

He also states that alternatively he could tear out the Marble (http://www.lightemprador.com) and (http://maplered.com) mortar but this will obviously cost more. If he does tear out the Marble (http://www.lightemprador.com) should the travertine be placed in a bed of mortar again vs thinset?

Any thoughts would be appreciated.