Caveat: I hate needles, and doubt I ever would do any optional surgical procedure without a doctor actually recommending I do so for medical reasons.
However, her reasons for having this surgery are essentially irrelevant. I doubt it was for medical reasons, or it seems it would have come out in the letter, unless that was editted out.
Rude is rude. It's one thing to compliment someone, and another to be crass. I don't know what generation the people responding to this are, for the most part, nor that of the LW & the ones commenting to her. However, "boob job" isn't normally a term one brings up to another person, unless you've discussed it before the procedure or you are discussing your own.
They could say she's "had something done," or "look different", or has "changed." I can't imagine commenting on someone's nose job, in so many words, and noses are literally out there for everyone to see.
Some people are just uncomfortable with attention, while at the same time wanting to look their best. A woman might cut her hair from long to short suddenly, because she's decided it would look better. However, that doesn't mean she really wants all the attention of everyone's comments. It's not rude to make those comments, but some women might find it unwelcome. Cosmetic surgery is much more effort than a haircut, but both can change a person's personal image.