I agree with Patrick: you should terminate the employee, immediately. Try to understand how lucky you just got here. Here's a simple tweak to the story you told that creates a nightmare scenario: the person who reported this to you, who was called a "milf" by the bizdev guy, was on a PIP.

I disagree with Patrick on the mechanics, subject to one condition:

If, like a sane person, you consulted a lawyer before you began hiring people, and so you're offering a standard employee agreement --- the kind every competent firm in the industry offers --- then the answer here is:

Just fire, and move on.

I don't know that "with cause" adds much.

The principle here is pretty straightforward. You got lucky this time. A new hire crossed a serious line, and the person they irritated came to you directly with a complaint, rather than through their lawyer. You will not get lucky next time. And next time, there will be a history that you'll be accountable for.