breast augmentation-is there a correct response

Alyssa
Ok.... walking through target with my sister (who is older, has a bunch of degrees and is known as the "conservative one")
She turns to me and says "I'm getting a boob job". I laugh. (She's always been an A and I've always been about a D)
She says she's serious.
I respond
"No! You don't need that, you are beautiful"
I'm told I didn't respond appropriately.
Is there an appropriate response?  I mean I get it, she had kids and she said her boobs are making her insecure.  I'm all for it but what if my initial response was "thank fucking god".  
So my question,  what's the best response to someone telling you they are getting plastic surgery.