...perhaps he is the project leader? just looking for ideas / thoughts.
There are 2 ways I know of sharing map development.
You can open 2 sessions of RMXP, and Copy / Paste from one to the other. (prefered)
You can also copy the Mapxxx.rxdata files into your games 'data' folder.
In this case, you need to create a dummy map in the master project, close the project, copy in the new map file,
then reopen the project. The advantage is, they only need to send one file, rather than the whole game.
In both cases, you have to be diligent about keeping your tilesets synchronized, and your events & variables, etc...
As well as any other database objects (enemies, troops, etc.) that you reference on your maps or events.
My best recommendation would be to do a lot of planning BEFORE you even start to build your game.
If you know the layout of the world, and exactly how many maps there will be, then you can divide them
up, assign areas to each person, and then choose one person to maintain the "master" copy of the game.
The more planning you do up front, the less headaches you'll have down the road.
Be Well