Hey, she knows about you; you should have known about her. You need to ask him why he feels the need to flirt with a stranger and find out what she's doing for him that you're not.You could even look at this as an opportunity to figure out what's missing in your relationship.
Simply create a username for your chat session and you're in the main chat.
Flirting can be fun, but some people may take it too far.
The great thing about the Internet is, if someone says something that offends you, you can simply let the person know you are not interested in further chats.
In most cases, people seek out attention from the opposite sex when they're feeling unappreciated at home. Still, regardless of what his motives might be, you need to tell him to stop corresponding with her ASAP.
It makes you uncomfortable as it should and it's certainly not helping your relationship.