Please Help My Friend (Female) Get Lean

Ok, one of my really good friends did gymnastics since she was 5 (She is 17 now). She was really built for a girl, but she wasn’t fat. Now she quit gymnastics about a month ago and she has gained a lot of weight (all fat). I guess it is because her metabolism went down and she was still eating as much as before when she exercised. She just got a membership to a gym so she will be working out regularly now, but she wants me to help her. She wants to lose fat and she doesn’t want to gain muscle (she actually wants to lose muscle but im not sure if that is possible). anyways, can somebody please help me with a diet plan for her as well as a general workout schedule (nothing too extreme because she is not the most dedicated person in the world.)
. if it was a guy i would have no problem helping them, but i have no clue about diet and workout exercises for women. also, do you think any supplements might be beneficial (MD6) ? Thanks a lot for the help, Nic