Underlayment should have been attatched with screws or staples at a rate of every 3 4 at all seams and 8 10 through the field of each sheet.
New tile floor squeak.
See our article for the 5 things that might be causing your floor to squeak and creak.
Use superglue for broken bottom.
In the case of old shoes squeaking may happen due to damages either to the outsoles or insoles.
Look for protruding nails and hammer them into the wood.
Was it set directly on the plywood.
It might be trying to tell you something.
Either way you can easily stop shoe from squeaking at home.
What kind of substrate was used for the tile installation.
I doubt that it is the tile assembly itself moving independent of the floor but that would have to be verified.
Gaps between the pan and floor sometimes squeaks are caused where the floor tile meets the shower pan.
How to stop shoe soles from squeaking for old shoes.
Normally there should be a slight gap between the shower pan and the floor tile.
Have your friend walk across the floor again to determine if this corrected the squeak.