That's how things go I'm afraid. At the moment your friend is caught up in the excitement of her new relationship and she's spending time with him and his friends - they know him and she feels she can get to know him better by mixing with his circle. Her old friends seem boring by comparison. Once the novelty wears off or her relationship breaks down, she'll be back looking for the comfort of the familiar. It is then up to you whether you want her back in your life. You can support her and be there for her when it all goes wrong or you can cut all ties and move on, it's up to you.