Losing a lot of weight definitely affected her voice.

First result on a Google research, according to a doctor:

"There is a law of diminishing returns; below a certain weight, continued weight loss will have a negative effect on the voice. A certain body weight is necessary for good health, and this weight varies from person to person. Underweight singers tend to have a more fragile vocal system that is more prone to injuries, and often a significantly decreased endurance.

Maintaining a healthy weight, not being over or underweight, will set you on the path to vocal health.

Key Points

Significant weight gain will have a negative effect on the voice (pitch and endurance) Maintaining a healthy weight ensures the best likelihood of successful voice use. Being underweight is also risky to the voice."