Tell your customer to get back to the installers, who should have taken this stuff off before they left - ask them to come back and refurbish the windows.
Then again, if they've signed the job off and paid, they deserve what they've got . . .?
Why don't you contact the installers for advice, Vince? Going by the style of frame, there's a chance that you'll find the manufacturers name, and date of the job, on the inside part of the frame, between the two sheets of glass.
Have fun!
Oh, and how's the back? Or should I not ask?