I am in total agreement with SeventhCrow...
"K and T obviously have major issues. The important point to keep in mind is that T is as much part of the problem as K. She's enabling all of K's drama, which indicates that she gets something from it--and she's likely going to try to get that sort of drama from you, should you get involved deeply.
It's not just an indication that their relationship isn't very good, it's an indication that each of them isn't in shape for a serious relationship."
That's basically where my thoughts were going. If she is willing to settle for a relationship that doesn't seem especially fair or healthy, there is a reason for that... and it is probably going to result in drama that you, your partner and your child do not need in your lives, especially as you're coping with the issues your partner faced following the move.