Estimate/carry plenty of gas....but more importantly, check the weather forecast, file a sail plan with folks on shore, take phones, plan for motor failure/alternate safe harbor/take-out points, etc.....don't get killed!....good luck!
This is what makes this trip all that more of an adventure.
WIth 3 people and a load of camping gear in a 14 foot boat, taking excessive fuel is a logistical problem.
It is Florida and the weather changes rapidly.
Flamingo Florida is literally at the end of a 40 mile road at the southern tip of the state.
Where they are going there will be no phone service, no alterate takeout points,
and finding a safe harbor will mean boating up a canal into the swamp where no one will find them for a long time.
Other than a rescue party starting a search if they do not return when expected, they are on their own.
Should they even consider making this trip? Absolutely!! This is the the stuff that memories are made of!
Because the OP is asking about fuel requirements, it shows he understands the dangers at hand.
Unless there is a lot of trolling in the plans, 6 gallons should be close to twice of what is needed.
Once he arrives at his camp site he can see how much fuel was used to get there, and adjust his plans for the return trip.
The good news is he will likely have a following wind for the return.