i would be telling him straight out how offended you are at his suggestion, it is not ok to tell someone how to disiplin there children.
you are doing the right thing, each child and each parent is different we all have to find what works for us.

having said that tho, im not perfect i use to smak my boys and it took me a long time to realize that it just dosent work, if anything it makes them worse, if ds1 knows he had pushed that limit, he would keep going just to see if he could get another. yes it did feel at the time like a reasonable solution, but it wasnt it just created bigger problems, and a bigger hole.
i was waiting for a bus with a friend and her kids the other week, she has 2 girls the same ages as my boys, the boys were being idiots, showing off (yes at 3 & 5 they show off infront of girls) i tried to get them to settle, knowing the bus was taking forever, and she mentioned i should give them a smack, i explained to her how it dosent work on them and i had stoped smaking them. (i literaly saw a lightbulb go off) "what if i gave them one? might shock them enough that they will calm down, works for the girls!" i did explain it again, but she couldnt understand how it makes them act up more, but did give up on the smacking issue!
your friend might give up and let you parent your way to if you explain your point/situation everytime he brings it up?