By your description, it seems that your customer wants to mount an acrylic box on the wall to cover the entire framed artwork. That may be a good idea, as it would provide some protection from environmental temperature and humidity changes, accidental damage as well as vandalism, and the cover would also make theft more difficult. However, it is important to note that there are no truly effective ways to prevent theft, since a determined thief could simply take a piece of the wall along with the art.
For the best view, use anti-reflection coated, 99% UV filtering Museum Optium Acrylic. I suggest contacting Small Corp, Showcase Acrylics, or another fabricator who knows how to build acrylic boxes out of this anti-reflection coated acrylic, which requires a special technique for bonding the edges. Such a proficient fabricator probably has worked with museums and valuable artworks enough to also suggest appropriate ways to mount the box over the art.
