What's the deal with that stuff?

There is a difference between changing a bad habit or attitude and changing who you really are. Drinking does not define a person and is not a trait that makes them who they are. This girl seems to be making a positive change for her health, and as long as it is something she genuinely wants to do, there is no problem. It would be different if she were dropping genuine interests or trying to be an introvert rather than a extravert for her boyfriend, but what she is doing is perfectly healthy. Relationships often change people for the better. I have become more mature and sensitive since I've been with my girlfriend :)