Dani,
Not sure what you/he means by "deglazing".
Automotive body putty will work to fill in depressions and voids in the cedar hull. "Glazing putty" in an automotive application is used as a follow up after the bondo work is done. The glazing putty fills in the sanding scratches and pinholes in the surface, before primer and paint is applied.
I have used the "bondo" quite successfully on many projects. I suspect there will be no need for actual "glazing putty". Bodo can not be feathered out to as thin and fine an edge as the glazing putty. Just depends on how smooth a surface he is looking for.
This is assuming he is looking to apply the bondo directly to the bare wood and then glass over top. If he is wanting to level/blend in imperfections after the glassing operation, before painting, then he may want to glazing putty.
As to using the products offered by the West Systems instead of the automotive products, either one should be just fine on bare wood. If the hull already has an epoxy coating, then the automotive product may not stick to the already present epoxy surface. My understanding is polyester products (bondo) will not stick to epoxy. But epoxy resin/cloth, can be applied over the polyester bondo.
I hope this helps and doesn't just confuse you. If you want yo talk over the phone send me a PM and I'll send you my phone number.
Take care,
Huntindave McCann
SHELL ROCK IA. ,,,,,, "As sailors grow older, the wiser ones move to smaller boats." Thomas Firth Jones, Multihull Voyaging