It depends (always my favorite answer
). ..Leyland cypress is a quick growing shrub that does well with being pruned. However
, when trimming its width, you need to be careful not to cut back into older leafless growths, as new growth will not appear from bare wood. Cutting that close to the trunk sounds like you might well be cutting into bare wood. That will leave a bare brown hole that won't grow back or fill in. You also need to check with local regulations. Different areas vary on the rules about what is ok to do to someone else's tree. Perhaps start by discussing the problem with the nice neighbor?