Would you?

If you're good friends with a guy, even though you've only known them since September, an even though you know he has a girlfriend but he accidentally text you "Hey babe" would you reply "Oh Hey babe?" or something else? 
And one time he even said he'd kiss that girl to make me mad and see if I'd really dumb him.. Out of all the girls he knows and he always talks about her. 
I'm a really jealous girlfriend and this happened with my boyfriend and one of his friends, I've never met her before. I don't want to jump to conclusions or anything and I hate being the "bad-jealous girlfriend". I hate knowing about how close he is to girls he just met. And they go to the same school but I don't..