There are a few reasons as to why your map is crashing for him. You might have used meshes with a high poly count (many triangles), in which the best option is to just remove or minimize the use of them. Do you have a weather cycle? Weather cycles can cause the game to crash as well. If you have lots of meshes with collisions that aren't really necessary, you might want to remove/reduce the use of those as well. Sometimes a high use of particles can cause map crashing, too.
If any of these might be the case, I would suggest removing/reducing the use of the creations causing the issues, then try re-exporting your map, and let them try it out again.
