I think that is a great step - even to have her hold the string, as well as the symbolic side of letting it go. If you get the chance to do it again perhaps under a more "controlled" situation that she can feel more comfortable, it might be another positive step. As a phobic of this problem, my worry would have been if it floated to the ceiling, how would the heat of the lights effect it - maybe it would break etc - just be careful that she does not become paranoid with smaller issues that others may not see as a threat. To me, sharp corners, even having them lay on the floor could all be potential issues. Good luck, and let us know how things progress with the Doctor.