Boyfriends ex girlfriend.

SO! My friend, (let's call him keegan) shared a post on Facebook today saying if he gets 500 comments then he would reveal his crush.. So I commented and we went back and forth then all the sudden my boyfriends ex (we'll call her Brianna) comments then someone else so I just kept commenting, trying to ignore her and before I know it Brianna is reacting to my comments and talking back to me.

Now a little back story about her, my boyfriend was with her for a few years but she was awful as far as he says cheated on him left her kids with him all the time she also cheated on her husband at the time with my boyfriend so yeah shes kind of a shitty person so needless to say.. I've never liked her and now that we've said a few words to each other on these comments I'm worried that she will think we're cool or maybe she's using me to get in close so she can talk to my boyfriend, also don't want to seem weird or crazy for talking to my boyfriendd ex girlfriend idk..i just want some advice on what you guys would do or think..