Probably the nicest answer would be just to say they look great. If they are comfortable with people knowing they had surgery, they may "spill the beans."
Plastic surgery is an immensely private affair for some patients. Many husbands have no idea their wives have had surgery before, and the wives make sure that I know this beforehand when I am talking to both of them together.
For celebrities and people in the public eye, their beautiful persona may rely on the illusion that they are naturally gifted or beautiful and have never "had work done", although people are far more open than ever about having had surgery.
As a plastic surgeon, I have had patients look at me in public and suddenly look away, as though knowing me would be evidence that they have had work done.
The surest way not to be embarrassed as a patient is to be honest, often in degrees. I give the advice that, if asked, patients may want to admit to having their eyes done, a truthful reply, and just not mention the facelift, LiveFill, cheeklift, browlift etc. That way they are truthful but haven't given out their entire medical records to a nosy stranger.