The panel will certainly come back of the wall by the looks of it, get a long sharp knife in there. The trouble will be with the corner strip, if it's siliconed in properly then they are a real bugger to get back out again.
Is the wall panel solid MDF/Ply backed or is it hollow board?
He has done a botch job as like others have said, he should have made sure to have checked the levels first and sort that out before whacking the boards up.
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