If a non-Catholic woman marries a non-Catholic man, then they get divorced, would it later be okay for a Catholic to marry one of these people?
The Church presumes validity of the marriage of two non-believers. Unless there were some other impediment (such as a prior marriage on one or both of their parts), there is no reason to assume that, just because their marriage "ended" in divorce, it was therefore invalid.
If one of these people were to want to marry a Catholic, that person would need to present his prior marriage to a tribunal to see if it were valid. If it, in fact, was valid, then, no, he would not be able to marry anyone else.