thats quite strange...
im no expert but
do you and your friend send the map back and forth to get work done?
maybe its a WE compatibility problem between the 2 of you? Have you tried changing the interface of a map that your friend has made?
If its just THAT particular map that has a problem, try deleting all the triggers, or even all the stuff from the map (after backing up ofc). with nothing on it, it should work with nothing possibly interfering....
I doubt its the problem, but just throwing ideas out there =/