That depends on what you two talk about.
Maybe .. as long as it doesn't bother their friend.
Is it wrong? NO - if you're trying to steal your friend's girlfriend.
If you don't trust either your friend or your girlfriend to spend time together, maybe they shouldn't be your friend or your girlfriend...
I don't think that it's wrong unless there is a deeper meaning. If you are feeling that it was wrong, then I would suggest not to.
How else is he going to sneak one in.
As long as he isn't flirting or putting moves on her, or hitting on her, it's okay.
The flirting and move-putting began as soon as he opened his mouth.
Unless he flat out hit on her, like, "go out with me." or "you're hot" or "you're cute", etc, or "if I was your boyfriend......"...... but we'll never know, because he never states that he did.
I have a lesbian friend who talked to her gay friend for hours because both their significant others were gone. Yeah, and I can assume both of them were hitting on each other like mad because that's what happens between men and women.