Can a US citizen file a fiance visa for ex-husband or ex-wife?

The short answer to your question is "Yes." The longer answer is that a significantly higher level of scrutiny may be expected from immigration officials to determine the bona fide nature of the relationship following a previous marriage and divorce, and this may include seeking evidence that the initial marriage and divorce were part of the ordinary (if perhaps unusual) pathways of relationships and had nothing to do with fraudulent attempts to obtain immigration benefits. It would be wise for the U.S. citizen fiance to consult with an immigration attorney who, after learning all of the relevant information, could advise about eligibilities, options and strategies, and who then could offer legal representation in the often complex application process.