Is It Illegal For A Minor To Chat Online On An Adult Website?

No, it's not illegal but it's not a very safe practice.

I don't know but that girl should stop for sure and not do that.  There are a lot of police and FBI monitoring websites like that trying to catch bad people.  Not that you or that person is bad, but that is why it is for adults, adults over 18.  And bad things happen all the time to innocent people.  Be very careful, please...