It is intriguing how the discolouration of the wood is more pronounced around the v-nails. I would guess that the timber could have been a bit green when the moulding was manufactured but I have never seen a stick bleed like that, even stuff which I have had on shelves near my ceiling which gets pretty warm in summer due to a tin roof and zero insulation. It is also unlikely in my opinion that the gas fire is an issue - most chimney walls would only be a couple of degrees warmer than the rest of the room and the rising heat from the fire would not be that extreme.
I wonder if the frame was hung in a hot spot where the sun beat directly on it for an hour or so each day or was subjected to more than usual ambient heat, possibly by being left in a car or a tin shed or some other unsuitable storage? I have also seen frames damaged by having hot lights placed too close to them.
I strongly suspect there is more to this story than the customer has told you.