Most Helpful Girl
I think it's normal if you really think he would leave you for the other girl...
If you are confident he would not do this and believe in the hopefully stronger relationship that you and him have...that sadly she may not have come close to ever experiencing.
Than what's to worry about...?
If anything she should be worried
Worried that he's never coming back to her...
Because he has you.
Just continue the relationship with him like you never knew that she became interested.
Be confident in your relationship...