Stans correct I rebuilt a number of items where the foam had turned to dust.
You really need to remove the armrest and see of it has a metal former inside, if so scrape out the old foam and using the outer cover to retain the builders foam that I used to rebuild my items.
Do not try to use to much foam at one go as it expands quite lot, and might distort the cover.
Build the foam up in thin layers, then trim the excess of the back when dry.
I found it was very successful except if the pieces being filled was thin with no metal former, which is how the component is fixed in place.
Brian
8227