Is there a secret girl code about not dating the ex of an ex friend?

Not me, but asking because one of my best friends has been hanging out with the ex of one of her former friends. She was friends with the ex as well way back in high school. The ex asked her out on a date and she thinks she likes him but she's not sure if it's a bad move to go on a date with an ex-friends ex. She asked me for advice but I feel like I'm bad at these types of things since I feel like it really isn't a big deal but she's freaking out like it is saying she's worried people will think she's a b*tch for going on a date with him so I wanted second opinions so I could at least give better advice.