If you wanted to test whether it's the new outfit that's the culprit, you could add his old one as an extra outfit and have her interact with him in it. There can be a slight lag with the attraction updating though.
Sims can be more or less attracted to other sims when they're wearing a particular outfit, depending on the clothing style and colour and their turn ons/offs.
I sometimes change the clothes to suit the partner's preferences, but I more often change the turn ons and turn offs to include their hair/colour/clothing style if I already like the sim's look. Or I just go with them falling out of attraction if I'm in the mood for drama