Welcome to HCM.
Weight gain, especially over a day or two, can be a sign that your body is holding onto extra fluid and your heart failure
is getting worse. Limit how much salt you eat.
As he is having CHF and cirrhosis
, ascitis and generalised edema are very common. It will be better if he take only that much amount of fluid that has been allowed to him. As it is difficult to say without examination, it will be better if you discuss this with the consulting doctor before increasing the fluid intake.
Hope I have provided the information you needed.
Wish you good health. Take care.