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so should she not tell him get lost I have a bf?
In an ideal world, yes.
In real life, it's very rare for a girl to be this straightforward. They will only do that if you really get annoying, or you get classified as a player/joke.
The best you can expect in most cases is the silent SPAM, when they ignore their suitor.
Second best is the "I enjoy the attention but I will not reward the guy with banging my pussy" behavior. It's essentially when the guy opens, they chat with him for a while, they don't let the conversation escalate to sex, then they leave like 5 to 10 minutes into it, and never open. Repeat the cycle a few times, until the guy gives up, or he becomes needy and gets the "I have a BF" speech.
If the advances are welcome, that's where you need to worry/react. She opens the guy, the conversations become flirtatious, she talks to him for hours. Worry. Call her out.
Beyond that, there is cheating. She meets the guy, and unless he totally screws up, she fucks him. Some girls will go as far as setting up dates and then flaking. If she sets up the date with no intention of going there, classify as "enjoy attention". If she sets up the date and you know she wants to go/is tempted to go, then next. It's only a matter of time anyways.
Only you can answer the question as to what your GF is planning to do. Up to now, nothing she did is worth writing home about. And it might be simply courteous behavior (she replied but totally ignored his offer to hang out, right?) where she prefers to act like it did not happen instead of being rude ("got a bf - get lost").
Keep an eye on her Facebook, and on her texts. Anything weird shows up, call her out. And be ready to next.