If he just wants to switch the glass for a vent window, That is easy... He can easily just remove the glass and swap it for the glass he wants with the vent window frame still in place.. Done it many times.. He can keep the old gasket and repair it (It usually comes out in two pieces from age)... or just install a new one....but a new one may need some trimming to fit correctly...
I like to secure the glass in and into the seal and frame using a very small amount of black urethane. After it dries, noone sees it and noone is the wiser and the vent glass will never go anywhere... yet can still be taken apart in the future if need be...
Just my experience...and 2 cents...lol...
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Tony K.