I'd say hide the scale. She may actually lose very little weight doign xfit. All that muscle replacing fat and getting better and compound weight lifiting movements do that to you.
To feed the OCD that most people have about scales, track your performance on your WOD and start a strict food log. In about 2 months review your progress on choice work outs.
You could even pick one of the body weight work outs and do a base line run today. And then instead of the normal wod once a week repeat that and see how much better you do.
Either way, ditch the scale in the bath room for the a food scale in the kitchen