You're also making the two basically faulty assumptions:
First, that the PFD will go off at all. If the foulies are well enough sealed to trap air as you suggest, they may keep enough water out to prevent the PFD from triggering. It certainly can't be manually triggered,
since that is not an option if the PFD is inside the foul weather gear—the manual release is not reachable.
Second, you're assuming that the PFD isn't going to be asphixiating him by compressing his chest and making breathing difficult. Also, the collar part of the PFD inflating could cause the collar of his PFD to strangle him if the foulies are zipped all the way up.
BTW, IMHO, if the FWG isn't zipped all the way up, then it adds NO BUOYANCY...
Third, in most situations they'd be wearing foul weather gear and a PFD, would they not also be wearing a harness? The harness obviously has to go outside of the foul weather gear so they can clip in... That would be yet another thing constricting their breathing as the PFD inflates.
Taking off your dungarees is one thing... since your pants aren't likely to strangle you... and filling them with are is SOP, even in the water safety lessons I've taken as a Boy Scout...
If the foulies are over sized -which they should be. Having a PFD under when inflated will actually gardner greater success due to buoyancy. You are making a statement that you yourself have never experienced. There is a reason why in the navy - if you fall over - you pull off your dungarees. Its to trap the air to make it a floatation device. The worst that comes from a PFD inflated over a restricted jacket is pro-longed buoyancy.
BTW, we're talking about a PFD inflated INSIDE a jacket, not over...