The idea was to then horizonally glue and screw boards on the back side of the raised panel so to give additional strength and to keep the raised panel from falling out
You can't glue and screw horizontal running grain to vertical running grain, because the wood will move, shrink and crack etc.
As proud as you are of your cabinetmaker friend I would think he should have know that. You essentially have to make doors that run horizontally using the stile and rail approach. Two outside rails with a center rail, an upper and lower stile.
To keep the panels secured, you can run backing vertically, then rout a tongue to fit in a groove on the rails and stiles and still have "raised" panels on the outside.
Your pictures leave me with a vague idea of what's going on, but there is not a lot of reference to go by.
It's apparently a seamless look from the outside with the verticals. I can just barely see some joints running horizontally. The interior panels should also have the grain running vertically. You can fasten the panels using a screw or mechanical fastener, but only in the center of each to allow for expansion across the width.
If you can separate one horizontal rail off just one side then slide your new panels in and reattach the stile, that would be the easiest "fix".
More photos of the details would be helpful and better lighting...not so dark.