unless you red maple is located on a downward slope from the neighbor's tree, the browning may not necessarily be from the stump killer. Any movement of the material would be downwards with rainfall, not sideways. If the material did move that way for whatever reason, there is little you can do as it is already absorbed. The only thing to do is wait and see. The damaged leaves will probably fall off and the tree may or may not send out new ones. You should keep the tree vigorous. If the summer is hot and dry, be sure to water once a week with 1" water and use a mulch at the base. This fall, consider fertilizing it to increase vigor.