Most Helpful Guy
No, unless you're enjoying it and you're flirting back in that case, you're becoming part of the whole thing. I'm gonna assume you're asking this because you don't want them to break up/you don't want it to be your fault so I say talk to both parties about this matter. Tell the guy that he shouldn't continue this and tell the girl (if you are aware that this is because of the flirting thing) that there's nothing between you two.
Most Helpful Girl
No it's not. Unless you actually made out with him or did something physical while knowing he is taken then yes it's your fault too but if you've just been flirted with then it's not on you.