The reference from the Watchtower magazine (published by Jehovah's Witnesses) which addresses this is on pp. 30-31 of the October 15, 2000 issue. The article is "'Questions From Readers', In the light of Bible commands about the proper use of blood, how do Jehovah's Witnesses view medical prcoedures using one's own blood?"

In part the article answers: 'The details may vary, and new procedures, treatments, and tests will certainly be developed. It is not our place to analyze each variation and render a decision. A Christian must decided for himself how his own blood will be handled in the course of a surgical procedure, medical test, or current therapy. Ahead of time, he should obtain from the doctor or technician the facts about what might be done with his blood during the procedure. Then he must decide according to what his conscience permits. (See box.)'

The aforementioned box reads: 'Questions to Ask Yourself If some of my blood will be diverted outside my body and the flow might even be interrupted for a time, will my conscience allow me to view this blood as still part of me, thus not requiring that it be "poured out on the ground"? Would my Bible-trained conscience be troubled if during a diagnostic or therapeutic procedure some of my own blood was withdrawn, modified, and directed back into (or onto) my body?'

So, yes some of Jehovah's Witnesses may allow the procedure as you describe without discarding their blood and others may not allow it. Your patient may benefit from reading the entire reference from the October 15, 2000 Watchtower so that she is aware that not discarding the blood is within the range of acceptable decisions.