Tell the truth, say it factually, say it minimally, don't explain at depth, don't argue, don't negotiate, don't criticise unnecessarily, don't attack, wish them well, leave / end the conversation, mute the notifications, turn your phone off or to silent, and go to an Al-Anon meeting.
If they're gonna be pissed off, they're gonna be pissed off, and no amount of tact will change that. Might as well do it fast, all in one go, and directly rather than slowly, in slices, and deviously.
My job is to get out of the deal quickly and cleanly and without muckying my moral copybook. Good, right, and nice don't always fit in the same jug. Prioritise good and right and sprinkle as much nice as you can muster over the top. Prioritise nice and the whole mission might fail.