I was thinking the same way you are. The concert has already taken place but, as we know, television can do many things such as mixing up live with recorded. My guess is the concert they show on TV will be the recorded one and possibly a live segment thrown in with Demi.
Either way, a big win for St. Maarten as many people across America will get the name of St. Maarten on their minds. Particularly because for some strange reason the island spends little to none on advertising its beauty and what it has to offer for tourists. This island is number two in the entire Caribbean for cruise ship stops yet both sides of the island do virtually nothing to promote its wares!