I failed the check but the romance continued. I didn't even get a Shadowheart disapproves notification. Albeit one can detect a soupçon of disapproval in the words: "if I need to step over your corpse to do this so be it"
Afterwords I said "I think it's wrong but I can't lose you" and the romance continued.
The part where you say you can't lose her comes before the roll. You ask her not to do it, she'll say she'll do it over your dead body if she has to, then you back down, and then Nightsong mentions the wolf which gives Shadowheart enough of a pause so Nightsong can give a speech and THEN you have your chance to persuade Shadowheart.