Stop in for a cup of coffee

Just had 2 employees go at it.. Funny thing was I was on the phone with one of them when this all happened so I heard it all. Also had another employee witness it... The one employee has had a grudge with the other for about 6 months now and wont let it go.. Today he blew up, threatened him to watch his back after work, then left.. This was unprovoked.. Someone was having a bad day and now made it worse for himself. Now I get the joy of dealing with HR and doing all this BS.