Display Name

Hey I wonder if I can get some advice?

I have a free plan and conducted an online interview.  After the interview, I was checking my emails and noticed that I received an email notification from zoom that tells you the person had logged in. The person used an offensive display name.

I did not notice this when the person first joined the meeting.  The person does not have a zoom account and denies that the display name was used. 

Any idea how this could have happened?

Many thanks