Well, the question is this: Are you dating him for looks or for his personality? People always tell you never judge a person by the book and is true. So if he had surgery to make himself feel better, bring his esteem up, or to enhance his features. Why should it matter? Is to make himself feel better, so he can be happy in his own skin. Some say they have surgery because they want to show their outer self how they feel in the inside. Just like people get tattoos to express how they feel on the inside. Is a problem if he has a body disorder, or if the reason why he got surgery is for low self esteem (if you have a low self esteem about your appearance, the no matter how many surgeries you get, you won't be happy since esteem is an inside issue than outside issue)
Is all good if small change and not drastic operation. Like girls that want big breast than their size or proportion then is bad. Is like too much a good thing is a bad thing... but this is drastic of course... All extremes touch. Whether which end it touches.
It's a personal decision, so the girl should understand that. If that happened to me and my new boyfriend looked drastically different, sure, I would ask a lot of questions. How come you did it? When did you do it? Are there any medical reasons you did it for. Afterwards, I would just give him a kiss and tell him how cute he is.