I think you have some excellent responses here dealing with the emotional side of this situation. As to the logistics of the original question - living together for financial reasons when the relationship is in in turmoil or even over, as well as a new guy in the picture for her...wow. I am in a vaguely similar situation - details differ, but I find myself still living with my STB Ex while I pursue a divorce. It is very, very difficult, and not particularly healthy for either of you unless there is a tremendous amount of communication, compassion and understanding there - in which case the relationship might not be where it is in the first place.

My recommendation would be - even though you indicate you are both still there for financial reasons - that one of you needs to find a way to leave. Stay with a friend or relative, find a very, very cheap apartment or room for rent on Craig's list - anything. Just because you both have a financial interest in seeing the renovations on the house completed doesn't mean you both have to reside there. I realize the alternatives are not appealing because while you still have the house it's difficult to afford anything that you would normally consider acceptable, but just remember that it is a very temporary situation - a short-term sacrifice so that you can both better in a better financial situation in a few month.

If there are literally no alternatives for either of you, then you need to have some very frank and honest discussion of ground rules and expectations of behavior from both of you while in this weird situation.

Best of luck, and sorry that things reached the point that they have.