This is gonna sound weird so I'm always criticising Carrie, but if they are really engaged, he's the one that disgusts me.
This is a woman that has clear mental issues and has been unhinged this year. She came out of a five year relationship where she probably had to beg for attention, for a photo together and she was dumped and felt like she wasted 5 years for someone that didn't want to commit with her. But has no issues commiting for someone else and to be open about his feeling and show how much in love he is just be the way he looks at his new gf.
Then Joel comes in, with his PDA, willing to commit and promises a life together and she jumps in because she's no longer willing to waste years before figuring out the guy is not the one. She wants the sure thing hoping it'll heal her.
She made the mistake of not going into therapy after the breakup. And I mean real therapy, not someone that just tells her that she's right. And the mistake of looking for a rebound.
But Joel is the sane one (theoretically speaking) that should know better and take things slow and not take advantage of her vulnerability. Hell, he's like a hyena praying on the dead animal while it's still warm.
Edit to add: she probably also told Joel about her problems with Ollie, this was another mistake. I learned this one myself the hard way: I dated a predator once that was 10 times worse than the ex before him, but he insinuated himself in my life by being the opposite of my ex that I complained about. Because he used all his knowledge to his advantage, he looked like a breath of fresh air to me, everything I had missed. Took me nearly a year to realise how much worse he was than my ex.
Tldr; Anyone that tries to show how much better they are compared to your ex(es) are often worse.