What nationality are they? If they are citizens of a Schengen country then "no worries", if not then a letter stating that they will be leaving on vessel so-and-so bound for the BVI signed by the vessel owner or master or charter
company is enough to get them through customs
, if they get asked at all. I've had friends fly in with paperwork and they were never asked - the only question they seem to ask
Remember that when you clear in to the BVI, where they will be leaving the vessel, that you need to have that noted on your clearance papers upon arrival (the officials will want to see printouts of the airline tickets for departing passenger from Beef Island onward).