What's the general rule about a girl "talking" to another guy?

You do not know the story behind her talking with this other guy. But there is no rules whether a girl is dating someone or not that she has to stay away from other guys. I once dated a guy who was obsessive of the fact that I talked to other guys even customers in the store I worked in. That was wrong. I had to talk to the male customers too as part of my job. He also did not believe my male cousins were just that. I could never mention them again. I would go ahead and ask to go to homecoming with you. Just because she chats with this one guy does not mean she is dating him. They could be just friends. And just being friends is not wrong.