If a coworker secretly records a private conversation and then uses that recording to complain to HR, is that considered a hostile work environment?

Hostile work environment usually means a condition caused by or permitted to happen by your employer.  co-workers who do things can lead to a hostile work environment claim if they are known by your employer and permmitted to continue w/o action.  What you describe is not a hostile work environment.  It's not illegal for your co-worker to record a converstaion (not over the phone) and then play it back. It may violate common sense of privacy but its not a hostile work environment.