Rob
Two points. Do you know how to "test" the strength of the yarn to see if Cotton or viscose? The most popular way is a bit "seat of the pants" but a skill quickly learnt. Take a length of yarn. Hold an end firmly in each hand. Ithen wet the middle of the yarn with saliva

( yes, in my mouth) Then pull on the ends until the yarn snaps. If it quietly breaks it's rayon/viscose. If it breaks with a reasonable SNAP sound, then it's cotton or maybe synthetic. You only need to do a few of these to be familiar with the difference.
The other thing is about marker pens. If it's on the foam cushioning, just slip a clean polythene carrier bag (inside out so no print transfer) into the cushion so that no transfer can take place.
BUT inform madam that there is always the possibility that the same has been done on, for example, the sofa/chair arm, and there is absolutely no way you can test for that. If permission to proceed given, ask her to sign that she acknowledges and accepts the risk should the problem arise. It's happened to me twice in 25 years, so it is quite rare.
Safe and happy cleaning

Ken