Anything more than a small amount of fluid, post-surgery, would be cause for concern, but of itself, not life threatening. The cause/nature of the fluid, and the presence or absence of symptoms such as
feverAllergic rhinitis
Coccidioidomycosis
Febrile seizures
Fever
Fever blister
Fever blisters and canker sores
Herpes labialis (oral herpes simplex)
Histoplasmosis
Malaria
Rheumatic fever
Scarlet fever,
pleurisy, shortness of breath etc., would be an important determinant of the urgency of seeking medical assistance. For example, if any of the symptoms just mentioned were present or if there were other signs of infected fluid, that would be a medical emergency. At the other extreme, if it were a small amount of fluid seen on a routine follow-up X-ray, in the absence of any symptoms, he could safely wait a couple days to see his doctor.
I hope that your friend was seen by his doctor yesterday and that, after a careful evaluation, the fluid was deemed to not reflect a serious complication. In this circumstance there are no absolutes as to treatment based only on the presence of fluid. One must rely on the physician’s clinical judgment, ideally on the judgment of the physician who performed the surgery, rather than an emergency room physician.
Good luck