My coworkers used normal tap water to water my Venus Fly Trap.
How can I save it?
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If this was a one-time thing and your plant doesn't show any symptoms of stress, you should be ok even without further meassures (speaking from experience...).
You might want to think about "flushing out" unwanted residue by watering over-generously with rainwater or filtered water (as you would normally use) and letting the excess drain from the bottom.
Another option before switching all soil might be just replacing the top layer, especially if the plant has been dry occasionally, but that's rather unlikely considering the preferences of a venus flytrap.
Changing all potting soil would be the ultimate resort if you suspect real damage to the soil like when an over-zealous co-worker decided that a generous dose of fertilizer would be a kind gesture to the poor plant...
Assuming it's not dead yet, you're just concerned that it will be affected, just water it with rainwater or distilled water or deionized water or whatever water you normally use for it, and let it drain, then water it some more and drain again, to flush out the minerals from the tap water.
I suppose you could put it in a container a little bigger than the pot and let it soak for 20 minutes or so before draining each time, but I don't know that it's needful.
If it's at work, a note for your co-workers might also be helpful.