I think some of us are too obsessed with being politically correct. he's not forcing her to change anything, it's a weird quirk of his. if it were a red flag, he'd be showing signs of irritation when she doesn't listen to him, or he'd have made it some type of condition to change her shampoo and not wear khaki jackets if she wants to date him.
he's just a weirdo which we love (I guess some of us don't judging from these comments, yall need to get with the program, sunbae is bae), and I personally think it makes their relationship funnier because of the way they react to each other. besides it is still his personal preference no matter what, the fact that he didn't stop hanging out with her even though she didn't change her shampoo just shows that it's not leading to toxicity.