If the tile is still attached to the substrate it is possible to beef up the plywood from below. I wouldn't be guaranteeing anything with the tile already set though.
But as mentioned you need joist spans, depths, centers and a real understanding of what is under the tile. Don't take the ho's word for it! He has already established he doesn't know what he is talking about. When he says 3/8 ply is he talking over the sub-floor or is that the sub-floor? (Unbelievable that would be)
But as mentioned you need joist spans, depths, centers and a real understanding of what is under the tile. Don't take the ho's word for it! He has already established he doesn't know what he is talking about. When he says 3/8 ply is he talking over the sub-floor or is that the sub-floor? (Unbelievable that would be)