I wear a headscarf full time. I would never take it off for anyone. Not even my husband. It is a request from God and that is more important to me than anything. And my husband has at times told me to take it off, though only out of concern for my health (it was really hot) or my safety. I have always refused.
If I was that girl, I would confront those other girls and ask them what their problem is. But I am 49 years old now and have worn hijab for a very long time. A young woman who just started wearing it doesn't need that.
What the boyfriend should ideally do is tell the friends it is not their business. He and his girlfriend don't tell them what to wear. If they have that big of an issue, they can choose not to see them. I would put the ball in their court. And then he should never tell his gf about any of it. He needs to stand up for her and protect her from such horrible people. Wearing the hijab is HARD. She doesn't need to know what happened. It will just make her feel bad and make hijab harder for her. Plus she may feel responsible for him possibly losing those "friends".