We recommend that a stuck photo not be removed from the glass. This can cause substantial new damage to the photo - which may or may not be repairable. The best approach is to scan the photo right through the glass in high resolution, then reprint it.
From a photo restoration perspective, it is far easier to remove any incidental glare caused by the scanner than to restore the new damage you may create when you remove the image. A severe tear across a face may be disastrous.
Even if you want to try to remove the image, please scan it in high resolution (300 dpi+) before you make the attempt. That way, if you fail you have a scan taken before you damaged the image.
I hope this helps. We see this issue all the time, and the scan through glass method only fails when the frame is of such a size, shape or glass condition that a good scan is impossible. If a scan is possible, try this route.
I have experienced the problem you are describing quite a number of times and being a Photographer and Framer I can see problem you are facing from both sides.
There are two method that you can try, first one is to remove the image undamaged from the glass and the second is to make a ch theopy of the image with glass attached the correct the copied image.
To try to move the image from the glass do this:
First mix up a solution of Kodak Photoflo solution 1:200 Clean water and immerse the image/glass with image on top of the glass, make sure that the image is covered by the solution use slightly warm water.
Leave in this solution for 24hours and check that the image is able to peeled off the glass(Gently).
If not leave longer and try again.
If this fails(It never has done that here) but remember that all images are not the same.
Some are fibre based but later BW and Colour images have a Resin coating on both faces of the image paper, the next step is to scan the image through the glass and then using Photoshop add some corrections and then print.
The final step is then to educate your customer in the prevention of this happening again.