Well that depends on who you are to him. For instance, are you all dating, talking, hanging out, or are you just his cool chick friend? Is he really just a guy friend?
If you think you're dating, does he know that? There are horrible horror stories about relationships that occur where one person doesn't even know that they're dating and the other person is picking out drapes. It's happened to me before.
Most of the time, our (guys) main mission is to procure the panties. Now, mentioning other women to a woman whose panties you're trying to procure is a bad idea. And in my experience, women use jealousy as a bargaining chip - men just get jealous. We try not to play women against one another because is reduces the odds of sleeping with either or both of them. And that is just not good for the continuation of the species.
You see, inherently, men believe in life and all the greatness that comes with it. So we aim to NOT reduce our abilities to do that thing that helps create life.
By the way, safe sex, kiddies. Safe sex.
Anyway, my guess is that if you're dude is mentioning another woman in front of you, he thinks you're just his friend.
Of course, if you're dealing with a "player" who believes that the best way to get more booty is by showing women just how much booty he's getting (the age-old women want a man that other women want conundrum), then it is possible that he would throw another chick around in hopes that you'd attempt to stake your territory.
That's totally a pun.
But my guess is if that were the case, you might know that since he's a guy friend and wouldn't be concerned about jealousy so much as getting played.
Either way, this all comes down to knowing the kind of guy you're dealing with AND being on the same page as far as status goes. My guess is that you're dealing with a guy you think you're dating or have expressed interest in. In that case, you need to make sure he knows this.
It was written.