My husband had knee surgery 5 days ago. He has fluid under his knee cap. Tonight, the other leg/ankle is swelling badly. Should he be seen at ER?

Maybe. No info about husband's age, medical problems, type of surgery, etc. The fluid under knee cap is probably not of concern UNLESS the skin over it has become red, hot, swollen, and very tender, or if fever. The OTHER leg being swollen is more concerning. If he has any calf or leg tenderness on that side, please go to ER right away to make sure no blood clot. Good luck!