A while back, we had a thread of Dom bringing Declan to Fleur and Bill's anniversary party, where Dec and past-Victoire's kind-of boyfriend didn't get along, and Dec didn't like how Victoire treated Dom. So that was the reason. His best friend, Henry, is more of a brother to him, and works in the department of games and sports at the Ministry. So she might know of Declan, but he originally was just Henry's study buddy, so to speak.
Even if the previous incarnation knew of him and disliked him, yours doesn't have to. I can reset him, to to speak. Declan already is kind of wary of Dom's relatives because of an NPC one that likes to mess with everybody's lives. But if he knows that Dom thinks she and Victoire don't get along, he'll probably be hesitant. Especially since Dom sort of hangs around and works at the Leaky with him fairly often, so she could wander in whenever.