I imagine he called that one not as a true prophecy, but as one of his regular everyday predictions. And that he made it while he was already in the walls. He was secretly watching everybody in the house, after all. He very easily could’ve seen that Dolores had a crush on the dude everyone wanted to hook up with Isabella and made a comment like “Oof. This isn’t gonna end well for Dolores.”
There isn’t much in the actual movie to confirm this, but I also like to imagine that between Bruno’s light spying and Dolores’s powers, that they managed to keep up an uncle/niece relationship even though the rest of the family thought Bruno was long gone. Bruno would know Dolores could hear him so it could be possible for him to just talk directly to her if he felt like it.