I don't get how that makes sense.
The story was that all three of them were in on it to facilitate their escape. If she confessed to Georgie as Narissa and admitted guilt, she wouldn't need to glamor herself as Faith for protection in the first place. For all she knew Georgie would kill all 3 of them.
After thinking about it, I believe She needs to.
For the plan to get into motion and attract Bigby, a murder must take place. This is her crime - puppet controlling a murder via Crooked Man's order and Georgie's hand. Of course she doesn't want to get killed by Crooked Man so her glamoring as Faith to set things in motion is important. Her confessing to Georgie is just a ruse and additional safety precautions.