First, if he sprays and kills the grasses, the clover will rebound. Second, the grasses eat up the excess N produced by the clover which is one reason they sneak in there to start. Have you ever seen clover die out in a yard without some chemical spray? Always stays prevalent along with a variety of other growth. Deer had the dog barking midnight last night as they ate in my front yard of 25 years. No they wont eat the grasses, but that clover will persist quite well. I see your thot, and get it, but it just not necessary for a plotter to worry of. Nature abhors a monoculture and will fight against it. Ive done as John is about to do thinking grasses were overwhelming the plot, but that was years ago and I would never do so again. No argument with me either way, but also just not necessary if one chooses.
But with that said, brassica will certainly love a planting in an rotated clover plot.