Hopefully this isn't redundant, since you've received many suggestions and comments.
They might be able to catch a ferry from Red Hook over to Jost. Pick them up in morning at Great Harbor dock. We did this with some family that were traveling separate so they could spend a couple nights on the charter boat while we were under way.
Not sure of ferry schedules, but did see ferry coming and going while in Jost last week.
The US C&I office is out of commission on St John so the Red Hook to JVD (via Cruz Bay) service is indefinitely suspended. All ferries to/from USVI to Tortola are currently via Charlotte Amalie.
