Final answer:
The nurse advises drinking orange juice daily to maintain potassium levels because furosemide, a loop diuretic used for heart failure, can cause potassium loss which can impact heart function and medication efficacy.
Step-by-step explanation:
The nurse would advise the client taking furosemide and digoxin for heart failure to drink a glass of orange juice every day to maintain potassium levels. Furosemide is a loop diuretic which can cause the loss of potassium along with the excretion of water and may lead to hypokalemia. As potassium levels influence heart function and can affect how medications like digoxin work, it's important to keep these levels within a normal range.