Wow, I am impressed. Excellent posts by Chip00, Bob, Planner, David et al.
As a mother, I understand your questioning. I also appreciate the fact that you are a very concerned mother, and seem to be very close to your daughter. You don't see that happening much these days.
I think with the research you are doing, the conclusions that you and your daughter are coming to, you are on the right track.
As an ex-resident of the DR, I agree with the posts above. Seeing so-called sankies in action, this is a typical behaviour of one setting up for the payoff. When the parents are involved, and are enjoying the relationship (however non-romantic it is) the sankie figures he is in like flynn. I have seen this countless times. I have also witnessed mothers pushing their daughters to explore these relationships, in which I have theories on why. 1)mother living vicariously through daughters' exotic fling

2) daughter moves to DR providing cheap vacations for family

I would say that it sounds like you (thank goodness) do not fall into this category. The precedent has been set though, by other families encouraging this type of behaviour.
It is also not uncommon for a "decent" Dominican girl to have a chaperone on dates, so for a "decent" North American girl to have one is something that shouldn't seem odd for a Dominican man, and he will work around that, by being on his best behaviour to gain trust with the chaperone. Once that trust is gained and the guard is let down, then the situation changes.
Since she is not interested in this "boy", there is no need to let her be alone with him. Continue on with being the chaperone, and if he doesn't like it, the friendship wasn't worth it in the first place.
Just my thoughts for the day!
Marlie