I didn't say that Ivy would manipulate Louise, I said that Ivy is the type [of person] who manipulates, while Louise is the type [of person] to be manipulated.
The evidence is already there in the game - Ivy has guys wrapped around her little finger; when she gets with Jeremy it's on her terms, and she can take him or leave him, she wouldn't become a slave to his cock. In the same way, after playing that act on Ed and getting Lena off work early, she points out how easy it is to manipulate guys. Essentially, Ivy sees most people (especially guys) as her playthings. Ivy keeps her cards close to her chest - I don't even think Lena truly knows her. If you don't expose who you truly are, and so don't expose your weaknesses, it's a LOT harder for people to manipulate or control you.
Louise, on the other hand, is a bit too naive, and too close to her emotions, while having little control over them - both which make a person very easy to manipulate. She assumes the worst of Stan (regarding her panties) without evidence because she doesn't like him, she assumes the worst about Ivy (as the one who she believes tempted Jeremy) simply because she doesn't like her, and she believed the best about Jeremy until she has irrefutable evidence in the photo Ivy shows her, because she doesn't want to believe that he's a POS. She also jumps into bed with Lena on the rebound simply because Lena is nice to her and there for her (and because she clearly looks up to her). She's very easy to manipulate - Lena could get Louise to do anything in her vulnerable state, if she wanted to.
It certainly wouldn't be out of the question for someone as socially-savvy and manipulative as Ivy to wrap Louise around her little finger over time if she truly wanted to and put her energy into doing just that - it wouldn't be easy or quick, but it would be possible, especially with Lena trying to have her happy little friend's circle.
That being said, Ivy would never do that, because Louise holds absolutely no interest or value to her (she considers Louise beneath her). She'd probably see it as being a pointless waste of time.