* The extent of the change: A significant change, like going from long hair to a bob, is much more likely to be noticed than trimming off an inch or two.
* How observant the guy is: Some guys are very observant and notice small details, while others are less so.
* How often they see each other: If they see each other frequently, the change is more likely to be noticed.
* How much he pays attention to her: If he's interested in her or pays attention to her appearance, he's more likely to notice.
* The guy's own interest in hairstyles: Some guys are more attuned to different hairstyles and might notice even subtle changes.
So, in general:
* Big change = more likely to be noticed.
* Small change = less likely to be noticed (but still possible).
Some guys might notice but not say anything, either because they don't want to say the wrong thing or simply don't feel the need to comment. Others might compliment her on the new look.