Pansy is in no way jealous over the hot pureblood girl being close to her future husband. Not jealous at all so freaking shut up about it, ok!?!? Hehe. That aside they would have lived together at school. Aside from the whole Theo thing Pansy does like her.
Lily Moon (Theo's estranged cousin) is a former Hufflepuff and probably saw Arden around at functions. If she is friends with Theo then most likely she would dislike Lily. There are opportunities for random meet ups and such. Lily also runs a shoe shop in Diagon called Muse so that is useful.
Justin Fitch-Fletchley may have passed her in the halls at school but seeing as how he is Muggleborn I don't see them hanging out anytime soon. Oh and Justin is quietly starting to form a revolution to overthrow the Ministry so there you go.
Remus may remember her from school and probably doesn't have an opinion about her but is not fond of DEs as a rule.
I think that's it. Feel free to say hello at any time on AIM at gothambookchick.