My 3 year old DD has very low muscle tone, didn't walk until 20 months and was in physical therapy for quite a while. She's a W sitter, as well. Her physical therapist never liked that she was a W sitter, although I know that there is disagreement about whether it is okay or not (I did it as a kid...). The therapist had us get a pair of these:
http://www.hiphelpers.com/
They're called hip helpers and they are basically stretchy shorts that are sewn together at the thigh. We were supposed to put her in them for 15-minute stretches a couple of times a day, for therapy purposes, not really to correct her W sitting all day long, just enough for her to develop some muscle memory for regular sitting as well as to help her sit without using the W position, and using her ab muscles instead. Obviously, you can't really walk well in these, so they are not meant to be worn for extended periods of time.
I, personally, wouldn't use these without a doctor or physical therapist giving the okay. If she truly does have low muscle tone, then mom should speak with the doc to see if whether there is something that can/should be done.
ETA: if she's doing the W sit constantly, then it makes me wonder if her ab muscles are very low tone and/or weak. There are muscle strengthening exercises that can be done to help those muscles. We did a lot of activities on an exercise ball for my DD to build up these muscles. But, again, I probably wouldn't do anything until she speaks with a doctor or physical therapist. Hope that helps!
