It wouldn't surprise me at all if Rover and Duji are dating, at the very least they're definitely still hooking up. No one in the history of dating has broken up with their girlfriend and been this close/such good friends with them. Something is definitely going on. Duji even said a lot of people in the office think that they'll get back together, so they must be giving some kind of signs.
Just think about all the stuff they've done together in the past 6 months. Gone as a "family" (Rover, Duji, and Gia) to both the zoo and an Elmo concert. Duji waxes Rovers back. Spent Easter over there, and if I remember he was over there for Thanksgiving and Christmas as well. Also a couple years ago he was in the delivery room for her birth. WHAT BROKEN UP COUPLE DOES THAT?!
They are ALWAYS talking/texting each other, he is always over there, they frequently arrive to work at the same time (which I think indicates one spent the night over the others house). Rover also has Duji in her will to get a third of his stuff.
And I found it interesting that when Rover threatened to post Duji's pic of her hairy vag, Duji said something like "if you post that I have lots of pictures of you that you wouldn't want me to post". I'm not sure if that means she has nudes of him, or if they have "family" pictures of her and her kid together or what.
And if they're not dating, there is no way either one of them will ever find another serious relationship unless they cool down their friendship. No boyfriend or girlfriend wants to date someone who spends so much time with their ex. Look at Rover's last girlfriend, Rover would say she would get pissed because he would spend so much time talking and hanging out with Duji.
But bottom line is there's a lot more going on with them then they are letting on.