Lupus can be a reason for the destruction of platelets. The positive ANA can be from either lupus or the ITP itself (it is an autoimmune disease itself). I've never gotten "bad" sick from it, and I do get monthly IVIG (which is a treatment for the condition) for my CVID, so mine is "controlled" under most circumstances. I've only gone low a few times in the last few years. It can be a "self-correcting" condition, and if it responds to meds (such as the pred), most docs do not want to cut on a person. Since your daugher's responded to the pred, I'd be inclined to go with the rheumy's opinion. That is just my opinion. I'm not a doctor. However, there is absolutely nothing wrong with getting a 2nd opinion from another hematologist and/or rheumy.
"There but for the grace of God, go I."
"... His mercy endureth for ever."