Rez is definitely still into her (I assume his OTT facial work is perhaps down to a quarter-life crisis and not having settled down yet at his age). It must hit hard seeing his partner of ten years, and the person he wanted to settle down, with moving on and incredibly happy (and splashing it all over her Instagram account) versus how miserable and uptight she become with him. Also, how many times did he use the same rehearsed response to everything Ruby said ("I really appreciate that"). I felt bad for the bloke, as he seemed very beaten down and nervous around her. I personally don't think Ruby still harbours any longing to be with him as she's clearly infinitely much happier with this Nick bloke.
As for Bella, I would probably react in a similar vein to Ruby tbf (minus the bitchy stares and sizing up of the other girl's outfit). If my ex's new date unexpectedly came up to me on a night out and introduced herself, I'd be taken aback and a bit flummoxed. Quite a bold move from Bella and something she should have done later down the line, seeing as she and Rez aren't even properly dating but "hanging out".
I also found myself really cringeing when Bella did that because of her bizarre, nervous body language (anyone else notice the erratic head bobbing and her voice nervously changing pitch?) I guess it was all engineered and encouraged by the producers, but still, you don't approach the ex of the guy you're newly dating like that
Also note how Bella threw Lily under the bus by telling Ruby how she bitched about her, then literally the next day started telling Lily all the bitchy things Ruby had said about her. On first impressions, she doesn't strike me as someone who can be trusted, but hey, we'll see.