Q. How much water can be added to grape juice before becoming Shehakol? If I am eating by someone Shabbos day and they add too much water, I am not being yotzi kiddush – does that mean I have to walk out? (They are running out of grape juice and they think it’s OK to dilute it.)
A. On question 448 we quote Horav Shlomo Miller’s Shlit’a opinion that one can dilute our common grape juice (not the light one), up to a bit less than half water and still make Hagofen and use it for Kiddush.
Horav Aharon Miller Shlit’a suggested that in your particular case, you may bring as a guest-gift a bottle of grape juice.
Rabbi A. Bartfeld as revised by Horav Shlomo Miller and Horav Aharon Miller Shlit’a