Yes TCBud, I would get rid of that plant and treat all the plants that aren't showing symptoms because broad mites are nasty. Regular organic mite treatments will not work on them. They are not like the spider mite at all. I've seen people recommend neem oil, insecticidal soaps, all the organic options, but in the end, I've never seen anyone beat them without chemical pesticides such as forbid.
And it probably wouldn't hurt to throw away the bucket and dirt, no way to know if they're on/in either without using a microscope.
I edited to add that if you do find that they have somehow migrated to your regular garden space, then Neoseiulus predatory mites will eat broad mites if you refuse to use pesticides. Predatory mites are hard to keep alive when released in the garden, and need high temps and rh, at least 80 degrees and 60 percent, for a few days while they take hold. If your friend won't use pesticides, you may recommend these to him as he has an infestation for sure.