I'm with
Jimayo on this one. The scene when they are in bed together in episode 5:
Jill: "Is something wrong?"
Bella: "No. Nothing's wrong."
Jill: "If you say so."
Jill: "It's not gonna happen again."
Bella: "..."
Bella: "I thought we had an understanding."
Jill: "We do. That's what I wanted to assure you of."
Bella: "I don't need to be reassured. I need you to stop talking about it."
Jill: "Wow..."
Jill: "Fine! But I wasn't though."
Jill: "And it's not my fault, I was going to sleep on the couch."
Bella: "Can we just sleep?"
Jill: "I'll take the couch instead."
Bella: "You don't have to do that."
Jill: "..."
Bella: "..."
That, coupled with Bella's thoughts earlier in episode 3:
Bella: "(Married women don't fool around with young students.)"
Bella: "(...)"
Bella: "(They aren't intimate with friends either...)"
Bella: "(But that...that was different!)"
Bella: "(It doesn't matter... I've stopped it.)"
Bella: "(Please, James... Forgive me.)"
Bella: "(Please, just come home again...)"
Bella: "(It's been long enough now.)"
Who else could Bella be talking about regarding being intimate with friends? Cathy? Jade? I don't think so. And straight into "Please, James... Forgive me." Who else if not Jill did she do something that upset James?
Sure it's not explicitly obvious, but there are certainly significant hints.
The only DIK that seems even remotely like he would betray them is Nick. He is Vinny's stooge, that seems clear, but I think he is being manipulated, and he's not directly intending to harm the DIKs.
His guilt over the trashing of Jacob's paintings is obvious. His directly reaching out to the mc to see what he can do to help with the mansion's repairs is clear. He was somehow unwittingly involved in the mansion trashing and he's totally realised it and wants to make amends.
So I don't see him as someone that would go around installing security cameras for Vinny.
The rest of the DIKs, I can't see it at all.
Therefore to me, Burke is the only other likely candidate. because he's obviously slimy enough, but who knows his motives...