Additionally, if he has loose stool with any of his constipation medications, he could lose electrolytes (like potassium and magnesium, particularly) which he is already at risk for being too low; milk of magnesia is another constipation medication which can keep his level sufficient and may help the constipation, though on the spectrum of medications we've discussed it's fairly mild.
As to risk of loss of excessive fluid, heart failure management is a delicate balance of fluid intake and output. He's on diuretics to prevent fluid overload, since his heart failure causes his kidneys to retain too much water, which then backs up in his lungs. But if he drinks too little water, or has diarrhea, or loses too much water from another cause, he can because dehydrated in terms of his blood volume, regardless of how much water has backed up in his lungs.
It's for this reason heart failure patients are fluid restricted and asked to closely monitor their weight (to look for increases which indicate fluid retention), but if something else occurs to disrupt an even fluid balance (like excessive diarrhea or vomiting), his kidneys can be injured from dehydration.
Heart failure is a complex problem! I'm sorry if this is confusing. Is that any clearer?