There really is no "fast" way to lose weight and keep it off. Weight loss is a combination of healthy diet and adequate exercise.
The problem with the use of pills, is that, while one might lose weight on them, the weight will sure come back as soon as the pills are stopped, often adding even more weight than when the diet was begun.
Many of the diet pills currently on the market can raise blood pressure to dangerous levels, and can also affect the efficiency of other organs.
Your aunt should implement a healthy diet and exercise program that would become lifetime changes.
Assuming she means fast instant weight loss as fast progressing fat loss all in a pill. THe leading effect of both prescription and over the counter medication is decreasing appetite. All the medications that increase metabolism by increasing body temperature are illegal in the US. Pretty much in people that choose not to watch their diets yet want to take a pill to lose weight, taking a multi vitamin will be effective. Most namely making sure she is getting all of her daily required amounts of vitamin B
If she is already tracking her calories, watching her nutrition, getting enough exercise, doesn't have pre existing cardiovascular issues, not competing in major athletic events, she could try the EAC stack. Pretty much it is Ephedrine, Aspirin, and Caffeine that have a synergistic effect on each other. As most commercially availible Ephedrine tablets already have caffeine, just add an Aspirin tablet to it. Ephedrine is now internet only after being briefly made to be illegal to sell from cardiovascular complications arising from people who should not have been taking it to begin with. However Doctors still prescribe it in suitable conditions.