NDA for witnessing fight at work

My wife was a witness for a physical altercstion between two staff members at her job. Nothing too serious, but there was a black eye as a result. She’s been contacted saying HR was going to reach out to ask her to sign an NDA about the situation. Is this legal? Happened in Kansas, for reference. It doesn’t seem like there’s any reason she shouldn’t be able to discuss the matter.

For background - she was caught in the middle of the two girls fighting in a small room and gave a witness testimony to a police officer after 911 was called. I would expect her involvement is done. It seems sketchy to now restrict her ability to discuss the situation as a whole, considering she was at risk of being harmed in the situation as well. From her account, both parties are at fault and laid hands on one another after a disagreement over using a phone. One of the two girls involved in the altercation (the one who got the black eye) is trying to make the case that “no one came to her rescue”, even though from my wife’s account she was the one that started the engagement.

Thanks for any advice!