It's
possible that Ruby might think that Amber corrupted Zii; I guess that if Zii manages to make a friend of her, say by bonding with Minew, being tactful about the yaoi, and generally being friendly in a cool way, Ruby might decide that everything was probably Amber's fault. But her default assumption usually seems to be that Amber's friends are part of Amber's world, and hence are likely to be tarred with the same brush. Remember all her comments and assumptions about Dillon and his "pervy" hands the night she arrived in the apartment.
Dillon has since got past her defences and made a friend of her, but even so he does have what she'd call a weird and over-active sex life (actually forgiveable because of the yaoi, hubba hubba, but that's a different issue), so she has no evidence yet that Amber's friends aren't all sex mad (terribly inaccurate though that is); it's just that one of them manages to be sort-of-okay despite that. Likewise, she may not be totally hostile to Zii, if Zii can turn on enough charm, but my bet is that she'll assume that Amber and Zii were probably as bad as each other, back in their teens, until given reason to think otherwise. Plus, not only will Zii probably enthuse about yaoi, making her definitely as bad as Ruby now fears that she herself is, but Ruby last saw her playing the rock star - and Ruby does tend to think in clichés a bit, I'm afraid, and we all know the clichés about rock musicians.
As to Ruby and Amber's pre-Zii relationship - no, we don't know what that was like. Probably a bit distant, frankly, given their age gap. But it looks pretty clear that Ruby
wanted to be friendly with Amber, at the exact time that Amber was getting it on with Zii.
She's made it clear that she was locked out of Amber's room when Zii was around, more than once, and she felt the exclusion; she wanted to join them in playing games, and the memory has lasted ten years or so, and by the looks of panel 2 there, she was still unhappy about it. (Call me a sentimental fool, but I find that strip terribly sad.) Ruby found herself shut out at precisely the moment she wanted to get closer to Amber, and she's only discovered why within the last three days. Whoever she blames for that, now or in future, is likely to catch a fair load of Ruby bitterness if they're not lucky.