You could try dodging and burn-in to mimic the lighting effects used by studio portraitists. They know a thing or two about making models look slimmer.

Tilting the enlarger head / lens panel will probably make things worse - unless she already looks like an alien. Apply the Scheimpflug rule to keep the image in focus if you try this.

Bending the paper on the easel (using a small lens aperture for depth-of-field) could produce some interesting effects, but again, hardly likely to help.

How about explaining to her that honesty and "telling it as it is" is part of your artistic vision. Then run for the hills…