Most Helpful Girl
They're most likely not trying to be friendly, I mean sure maybe they are, but this obviously has to do with your ex.
Your ex was probably too afraid/embarrassed to confront you. Her friends are probably covering up for her and trying to make you want her back.
Or, unlikely though, your ex might be getting her friends to test whether you'd go on a date with her own friends to see if you're honest or something? Once again, probs unlikely cause you guys are history and she has no business in your love life anymore.
I'd take up on their offer though.