Take her aside and ask her where Parker is?

You approach the large group and say, "Excuse me Mary-Jane, can I talk to you over here?"
"Oh-uh sure." She says a little confused. The crowd disperse as you take away their center of attention. "What is this about?" She asks.
"I need you to tell me where Peter Parker is." You says encouraging her mentally to tell you everything she knows.
"Oh I'd love to but I don't know. Why don't you ask his girlfriend Gwen Stacy." She says with disdain in her voice.
"Ok I will. Do you have some kind of problem with Gwen Stacy?"
"She's fine I guess, I just don't know what she sees in him. I mean, he could be with me." It seems you may have loosened her lips a little too much.