We visited there a few years ago on a week day and no one was at the church there. I wandered through the graveyard which is very well kept. If you let me know who your grandparent is I may have take a photo of the gravestone as I took many photos.
The address is St. John's United Methodist Church on Stuart St. in Elmont, NY. The phone number that I have for it is 518-285-6003 if it is still valid.