I think it's okay as long as
(1) a reasonable amount of time has passed since the breakup -- 6 months at the least, more if the relationship was fairly serious or long-lived,
and (2) the girl pursuing the ex-boyfriend gets permission from the ex-boyfriend's first girlfriend beforehand.
I know from experience that I wouldn't mind my sisters dating some of my ex-boyfriends from years past, but if it was a really recent ex-boyfriend I'd be pretty angry.
Granted, I have an out-of-the-ordinary view on this because right now I'm dating one of my ex-boyfriend's best friends. However, it's a unique situation because I am really good friends with my ex, he and I broke up a long time ago (almost four years), and the guy I'm dating didn't become friends with my ex and didn't know me until long after my ex and I had broken up. It's still really weird and sometimes awkward for everyone involved though, and I think it could definitely get messy if we hadn't talked through it as much as we had.
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