I had same on a lazy boy couch of maybe 8 years old. Fixed it this morning. maybe 1.5 hours. Tools. Staple gun, small bit flat screw driver, needle nose pliers and regular pliars.
- take off back cushion.
- at the back/top corner of armrest take staples out that connect leather and black covering to wooden frame to make opening large enough to get arm in.
- stuffing i found was a layer of memory foam and then white stuffing material in a sheet.
- memory foam seems to go on bottom and this sheet on top
- basically need to slide them together back up into position
- on mine was a cardboard piece that seems to supposed to hold the foam in place but had bent and just hung down inside instead of providing support
- didn't do anything about this so likely not a permament solution but hopefully can get 6 or 7 years out of before moves again.
- physically hard but not too complicated if take time
good luck.
L H, February 2011