There would actually be less pressure on the glass at 12.5 with the new dimensions as the pressure is distributed throughout a larger surface area.
WRONG! WRONG! WRONG!
Pressure is a factor associated with depth, not surface area!
The glass is not rigid, but flexible; hence the bowing of the front pane easily noticeable on any standard 55 gal tank which by-the-way is 22" high and almost always 5mm tempered glass. It is this "larger surface area" which is of immediate concern due to the amount of flexing or bowing that a 42" length of non-tempered 5mm glass will do! This aspect of the design is exactly why everyone has expressed concern and why many have recommended center bracing it. As for the bottom being only 5mm also. Easy fix...
Get a depth guage and find out EXACTLY how recessed the bottom is from the bottom of the frame (if it even has one). A set of highly effective and inexpensive braces may be made with 1/4" slats of acrylic epoxied to the underside in concentric rectangular pattern or X pattern cross the center with a few parallel strips (V's from each of the sides) for added piece of mind.
The height of the strips should be just a hair less than the recessed depth measurement taken earlier in order to accommodate the epoxy fill.
And FYI... At 48" long (6" more than the tank of concern), a standard 55gal has 5mm tempered glass on all sides and bottom glass too.
A standard 40B also has thinner bottom glass than the sides and again...Tempered.
If he used tempered, ok... salvageable... if not - it's a dangerous setup - especially for the occupants.