most of the time I match my top with my bobbin thread. If I have trouble with tension, I try to match a slippery thread (glide, magnifico..never used rayon) with something less slippery but close in weight..so maybe with a light weight thread, I would use SoFine. Most long armers find what works in their long arm by trial and error. You might suggest she check out Jamie Wallen's You Tube video about tension. I found it very helpful when I bought a pre-loved longarm and got no new owners class. Here is a link:
http://www.bing.com/videos/search?q=...A3EF3279102A80