Actually, I'm not so sure they can fire her. Some States prohibit "discrimination" in employment practices for engaging in lawful activities off-duty. I don't know if Ohio is one of those but azeem's girlfriend may contact Ohio's DOL and inquire.
Azeem, if your GF was passing the tape around work or sharing it specifically with her co-workers that is inappropriate and firing her was lawful. If they fired her solely because they became aware she made an adult film, that may be another.