This could be anybody on that pic. Black nails don't necessarily mean it's Louise. Girls often change their nail color. It could be Minerva, it could be Cindy at Wade's birthday party, it could be Ivy or literally anyone. Although I think that the most obvious option (Louise who likes black color) is wrong and Eva just teases us with these dark nails. Louise had zero interactions with Ian. She is either completely loyal to Jeremy (thus having no reason to cheat on him) or completely broken by his betrayal. In the second case, I don't think she would have a reason to attend any meeting at which Ian is present. Also, Louise is an extremely emotional person and she isn't the type to coldly plan revenge on her boyfriend with his friend.