Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
1
Is any of this happening now?
Call 911
Treat any suspected stonefish sting as an emergency.
Leave the water, begin safe hot-water first aid, and call the local emergency number or poisons information service.
Call emergency services immediately if there is:
Trouble breathing or swallowing
Chest discomfort or an abnormal heartbeat
Fainting, confusion, severe weakness, or collapse
Repeated vomiting
Rapidly worsening swelling
Pale, blue, cold, or numb skin beyond the sting
Uncontrolled bleeding
A sting affecting a young child or someone who is medically frail
After emergency treatment, arrange prompt medical review if pain or swelling is getting worse, movement is limited, numbness persists, or you think part of a spine remains in the wound.
These are follow-up concerns rather than reasons to ignore the original need for emergency care.
Other emergency signs, for anyone
Someone who is struggling to breathe, gasping, choking, or cannot get words out
Someone who has chest pain, tightness or pressure lasting more than a few minutes
Someone who has sudden weakness or drooping on one side of the face or body, slurred speech, or sudden confusion
Someone who is unconscious, or cannot be woken
Someone who is bleeding heavily and it will not stop with firm pressure
Someone who has had a seizure that will not stop, or does not wake up afterwards
Someone who has lips or skin that look pale, blue, grey or blotchy
Someone who you believe has taken an overdose or swallowed something poisonous
The article also says
Increasing redness, warmth, pus, fever, or a wound that is not healing should also be assessed.
Every sign above is quoted from the sofidex article on Stonefish sting. If you are not sure, err upward — it is always reasonable to be checked.