Eh. Hard to call it, really. Not enough info.
1.) If they were married in 2014 and separated in 2015. They've probably just been doing their own thing this whole time (hence, the Big Sean interactions). That doesn't necessarily mean that this was going on behind Dot's back. I've known couples that were separated for 5 years before finally getting around to filing for a divorce.
2.) You're correct in saying that she'd do it to Sean if she did it to old boy, but that's under the assumption that Sean is planning on seriously dating her like that and once again, we can't assume that they separated AFTER she'd been dating Sean or that we know what caused the separation.