INAL, but here&#x27;s what I think<p>&gt; I&#x27;m really upset by this guy&#x27;s <i>behaviour</i> and I want to fire him immediately.<p>If you are in the US, which that u in behavior suggest that you might not be...<p>If you have an established HR process you have to follow it.  Otherwise fire him discreetly and then let people that hes moved on in a few days.  Don&#x27;t tell either him or the employee that came to you that it has anything to do with their interaction and if he asks specifically during the firing process turn the tables on him and feign unawareness and ask what the hell he is talking about.  
I wouldn&#x27;t out her or acknowledge that you are aware of the incident to him at all.
If the employee who confided in you asks if that why he was fired, just tell her that its a confidential matter and you really can&#x27;t go into it.
I live and work in at will state and wouldn&#x27;t bother to lawyer up if I went this route (though it wouldn&#x27;t be a bad idea), but I would wait til I was calm enough to leak that he had done something really upsetting.<p>If you want to make an example out of him (or keep him around) you definitely need to lawyer up.