I don’t like his girlfriend

My best friend has been with his girlfriend for over a year now, and he only found out recently she cheated at the start of the relationship. My heart breaks for him because of how hurt he is and he is still with her, which i don’t mind i respect whatever he does i just don’t really like her as much as i did, and i don’t want to tell him that because he said he doesn’t want what she done to change my view on her, but it has