I would lay out MY BOUNDARIES, not ultimatums to HER.
And if that caused "resistance," then so be it. Boundaries, by definition, are markers of personal integrity that you CANNOT ABIDE due to your own moral code and emotional health. So, they are either dealbreakers or they are not. Apparently, her continuing to carry on with this guy, while she lives with you, is not a dealbreaker, and that's fine. Only YOU can know for yourself what those are -- we can't tell you what they should be.
I personally could not do that for any length of time, and "causing resistance," quite honestly, never entered my decision-making process once I got past my very earliest days of DBing. Instead, I always strived for "Do the RIGHT THING in every situation; the thing that God Himself would want me to do if He were standing right in front of me."
