This is true and the fact that she's contacting him and saying "I miss you" kind of alludes to the idea that this is a bad situation that he would be getting into with that choice.

Honestly, I have no idea why an attached girlfriend of another guy would contact another male that knowingly has feelings for her and tell him she misses him. That really really really does not sit well with me.

Please listen to that advice divine has given, run far away from that woman whether or not you pick the new friend to contact again or not.

"Bad choice", doesn't even begin to say to the extent I think it would be one.