Must have been a real good salesman lol. Shake roof should have had at least 20 more years in it if installed properly.
This is a tough question, I have quite a few steel tile roofs in this area as we have a local (Orillia) manufacturer Greg steel shakes. In any case from my experience the steel tiles are very lightweight in comparison to a shingle, do not conform to shape like a shingle will and that is what causes the cracking when asphalt shingles are applied as second layer or over wood shakes. In addition a proper shake roof may not have continuous sheathing but can be installed on spaced planks allowing the back of the shake to dry out. Shake roofs are designed to shed water not be water proof while steel tile when installed properly should make a water proof roof.
From a weight standpoint it should not be a concern. If the old roof was a plank style the shake may have been the only way to obtain continuous attachment points for the steel tile. The air space between the wood and the tile should allow any drying and humidity control issues to be dealt with. In short, weight, and provided the tile is installed correctly moisture are not issues.
Without viewing the attachment system I would defer only to the manufacturers installation guidelines and warranty as to the correctness of installing it over a shake roof.
If the manufacturer will warrant the tiles installed over a shake roof then the only real issues are what the condition of the shake roof was so as to allow the attachment required for the steel tiles and the condition of the framing and that you could only determine from inspection in the attic.
Were it me if there were ANY signs or rot or appreciable past leaking I would recommend the client have the roof framing inspected for rot especially around lower components as in where the framing crosses the header plates, soffits and sub fasica.