If the bottom line question here is, can you legally be disciplined for a conversation that took place outside of work, the answer is yes, you can legally be disciplined for a conversation that took place outside of work.

Does it matter that my supervisor and the employee in question are both Hispanic? Or that he is male, she is female, and they are constantly flirting with each other at work? Not in the slightest unless you are prepared to provide valid and supportable evidence that if you were also Hispanic, you would not have been disciplined.