It depends on whether you believe the manufacturers or the nervous. They claim that the weedkiller is inactivated on soil contact. The nervous say that yes maybe it is, but it takes longer than they would have you believe. Do you really need to spray now? Can it not wait until the potatoes are harvested? If your neighbour has sprayed the plants, they should die anyway without you needing to do anything. If you do decide to spray, then be very very careful about allowing even the smallest amount of spray to drift onto your potatoes. The weeds will survive repeated drenchings, the cultivated plants will die at the least smell of weedkiller. If the weeds are a real problem I would suggest cutting them down now to ground level, then used weed killer later when the new growth comes through, and then again when the foliage begins to die back for Winter. There that should cover every eventuality.
