If they failed to pay the fine (or contest the ticket), their right to operate in Florida will have been suspended and, as a reciprocal matter, in most other states. It is unlikely that they will be denied entry or arrested.

But, it is very likely that they will not be permitted to drive in the US and, if stopped by law enforcement for some other reason, they will be subjected to a much more serious offense for operating when their privilege to operate was suspended.

Easy solution for someone who can afford to fly here is to pay the stupid thing and be done with it.