I don’t find any problem with it. Just smile and say “Amen”, you are receiving a bracha. I was always taught not to take a bracha lightly even a “birchas hedyot”. Even a “hedyot” has the power to give brachos, especially at a time when he is in the middle of doing a mitzvah (simchas chossen vkalla). Always answer Amen when someone gives you or someone else a bracha.
In terms of actually saying “IY’H by you”, If everyone would look at it this way it would be fine. But limayseh many people are hurt by it so perhaps it’s not a good thing.