Having RSD means that she has nerve pain. Nerve pain expresses itself (among other things) in sensitivity to touch (even light touch).

If you use water on her foot, try and stay away from extremes in temperatures, stay away from ice-cold temperatures and stay away from heat. Normal warmth, normal coldness. And trust her on whatever she can tolerate! Water is NO treatment in itself, she needs medical treatment, but in the mean time... whatever alleviates some of that intense pain... do it.

Most of all, don't push her! At all. She will tell you exactly when too much is too much! And with that kind of pain: TRUST her! PT alone without some form of medical treatment or pain alleviation to be able to do PT is torture.