In all kinds of photography designed to flatter people, there are tricks to learn to tighten flesh. Even your basic 18 year old Playboy model, is posed with her torso turned to tighten skin above her hip bones. Generally bodies have to be turned slightly in every "joint", meaning neck, waist etc.People also have to maintain good posture.
In available light, concentrate on keeping the chin up and using a slightly higher camera angle.
Often slightly "fleshy" people aren't great to use as models because they've seen themselves in photographs before and are self conscious and self censoring. However if you do a great job, on the initial pictures, she'll be better on the subsequent ones.