My 300E has just started to do the same thing. fortunately, My dad owns an upholstery shop! If the rest of your armrest cover is in good shape, save your money because this can be an easy job. My dad says that finding the plastic like cording may be difficult but he has some cording that can be died to match easily. Replacing this is easy.....there is a zipper on the back of your armrest for removing. you can then turn it inside-out and razor blade the stitching apart. If you can find a replacement cord any nanny should be able to stitch it back together. You may however wont to make sure her equipment is up to date with a heavy duty sewing machine(ie: pfaff, singer industrial) otherwise it will not be able to make it through two layers of Leather plus the cording.
1957 Dodge D100
1967 VW Microbus 21 Window
2004 Beach cruiser bicycle
"It cannot be emphasized too strongly or too often that this great nation was founded not by religionists, but by Christians; not on religion, but on the Gospel of Jesus Christ. For that reason alone, people of other faiths have been afforded freedom of worship here." Patrick Henry 1776