Idk. In the case of Harley and Ivy, recent comics and 'The Batman' series aside they last appeared together in a comic in 2005. Hardly a stretch. Even if it was further back(the Harley Quinn series, 2003) I don't think it'd be a problem. Harley has never really been shown to hold back from feeling someone is a friend with little interaction and Ivy's only ever really been showed as accepting Harley as a friend so I doubt she'd let that go. Put those together and it's believable.