What is the photo mounted on? What are you floating it onto?
Rag board onto fabric: Frank's Fabric Glue
Alpha onto paper mat: Frank's Fabric Glue, or Yes past.
And if the photo was mounted on cardboard or masonite, it doesn't matter what you mount with, the damage is done.
Even white glue or corner weld will mount a mat board to a mat board.....
The critical mounting has already taken place. And that is out of your hands.... You might ask them (if you can't tell by looking) how it was mounted and on what. If they don't know, it probably didn't matter to them to begin with.....
So, it may sound cavalier or "un-professional" or non-FACTS, or NON-Certified, but the answer is now moot. Mount it so it works and is least envasive.
baer