Look for signs of it disintegrating, that is when it is done for. Up to that point, leave it alone, consider maybe even quarantine in bowl or something to keep anything in the tank from bothering it. A torn foot isn't always a death sentence, but it does take good husbandry to nurse one back. I would be curious about what caused the issue to begin with, maybe take some steps to avoid that again. GL.
Happy Reefing!