Ignore it if it was not said to your face

 

Otherwise:

"It's very rude for you to say that."

"Whatever the problem that's in your life, I'm not it.  You need to find out what's the real cause that is bothering you."

"God is watching."

"You want me to feel bad about myself.  How sad is your life?"

 

Of course, you need to discern his intent.  If he is looking for trouble by provoking you, escalating the situation should be avoided.

Your daughter may not need to say anything to that boy, other than "You have sharp eyes," the aforementioned retorts are for grownups to a grownup.