Could be a number of reasons:-
1. It could be a general comment. Nothing more.
2. Some girls try to make the guy insecure by asking about his friend and stuff. Best way I've found in this case is to act normal and be cool as if you are not affected by it. Or even better tease her "Let me set a date for you" or something similar on that line. She'll immediately back track "No No I was just saying ..."
3. Accept that we human beings are attracted to more than one person. Deep down all of us want to fuck/have as many beautiful women as we can. Same with women. Its just sometimes your inner desires show off in your behaviour.
So, Have some self belief in who you are. If she's asking about him she's not necessarily gonna fuck him too. And even if she does or wants to, good for you that you know that she's not the kind of woman you wanna spend your time with.