Brief Answer:
multiple possibilities mentioned.

Detailed Answer:
Hello and welcome,

I understand the query.
If she is gaining weight very fast and has fluid all over the body then there are a few possibilities.
1. she has a heart problem and the heart is not beating well enough leading to fluid retention. Most of these patients have breathlessness and cannot do their routine work as they feel short of breath, tired have palpitations and other heart symptoms. So please check if she has any of these.
2. she has low proteins in blood. Proteins cause the fluid in the blood to stay in the blood vessels. If protein is low the fluid leaks out side the blood vessels and collects in tissues leading to swelling and rapid weight gain.
3. similar fluid retention is also seen in women with low hemoglobin or anemia.
4. another common cause is kidney disorders as the kidney cannot filter toxic products and they accumalate.
5. another reason is hormonal imbalances especially low thyroid hormone leading to a condition called hypothyroidism.

Another condition is called premenstrual syndrome where women tend to gain weight during a certain phase of the menstrual cycle and then it gets relieved after periods.

As of now,she must see a physician and get evaluated first. She may need certain tests like complete blood picture, hormone testing as well as evaluation of the heart and kidney to help pinpoint the cause.

Once the cause is found then treatment can be initiated as per the cause and she will be alright.

[ please make sure that she is not over eating and gaining weight as this is not mentioned in the history].

Hope this satisfies your query.
Thanks for using HCM.     
Feel free to ask any more questions that you may have.
Dr Madhuri Bagde
Consultant Obstetrician and Gynecologist     
[You may also ask me direct questions at the following link. http://doctor.healthcaremagic.com/doctors/dr-madhuri-n-bagde/66844 Thanks!!]

---