Please do not PM me next time. If I have time and think I know the answer, I will reply to your topic eventually.
Anyway,
Other things comes to mind:
What build of the game are you (both) using? Check the number in the bottom-right corner of the start-up window.
Double check if you downloaded everything for that session. In other words: Check in Content Manager if you have missing dependencies.
Maybe the person you are playing with is using an illegal install of the game and somehow managed to get assets flagged as "not locally modified". This will confuse your game. Try testing this with someone who 100% sure has a legal version of the game with a small MP sessions+route.
Bit of an overkill solution (in amount of work), but you could try a fresh install and only download that MP session you are trying to play. If that does not solve it, it probably is caused by the game the other person has installed.