What kind of diet? If she wants to lose some weight, cut back on calories about 10% maximum, and add a bit more protein. If she has just started tennis she is burning more calories than she was, so going on a strict, very low calorie diet will only result in her being tired, losing muscle mass, and then quitting! Grab her by the *** and tell her to eat sensibly. Cut out the junk food. Cut back on the pasta a bit.
"Love is the irresistible desire to be irresistibly desired."-Frost