I agree with the above post by LiliRose. I think it was always inevitable that Jack & Ronnie were going to divorce, Ronnie had told him to move on before she took the stand, and refuses to see him in prison so I feel it was always on the cards. I think it'll be more interesting if Ronnie ever did return anyway rather than everything just falling back into place.
Ronnie's been behind bars for a few months now though and this is the first we've heard of it so maybe she's been thinking about it for a while but not wanting to hurt Jack. However, it seems Roxy goes to visit Ronnie in prison (not sure if this is the first time or not?) and Roxy will obviously tell Ronnie that Jack won't let her see Amy and maybe this fuels her/ gives her the courage to mention wanting a divorce. I did at first understand Jack's reasoning towards Amy and the whole custody thing but I think he's got a bit of a cheek not letting his daughter see her mother, and also because he's only just remembered he's got a daughter. I know it's not really a dig at Ronnie, but I do feel he's being really insensitive by basically taking away Amy from Roxy, when he knows that his wife is behind bars/ a broken woman because her children have been taken away from her. I'm sure Ronnie would be really upset when she heard what Jack was putting Roxy through and Roxy knows Jack is still totally in love with Ronnie so of course she's going to want to hurt him by telling him the news, just like he's hurting her. I would love for Jack to have a conversation about Ronnie wanting a divorce with someone like Dot or Max and for them to be like, well what do you expect and for them to point out that he's doing to Roxy, effectively what Archie did to Ronnie (although obviously slightly different).
Just because they divorce, it doesn't mean Ronnie won't come back. I always said if she does come back, it'll be for Roxy, not for Jack.