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.