It's possible, if there is enough wind, and if he is careless about how he sprays.

I spray roundup around my own fruit trees and veggies and have never had a problem. I spray when there is no wind and keep the sprayer nozzle right down on the plant I am after. Plus I stay back away from my fruit trees.

I suggest that you very politely tell your neighbor that you are worried about drift and very politely request that he not use herbicide any time that there is wind blowing over towards your veggie patch.

You could offer to give him some ripe tomatoes, if he doesn't have his own veggies.

I don't know what he is spraying, but most herbicides don't have any residual effect. Sunlight neutralizes them.

Plant disease is different. That can get into your soil and wait there.

I have no experience with this, but wonder if you might drape some old sheets or light tarps/plastic over the plants while your neighbor is spraying. Would be on the safe side.

The 2,4-D type herbicides are death rays for tomato. And they can drift for miles under certain conditions.