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Old School really shouldn't cause any compatibility errors.
My first instinct is to check you are both using the same version of the game. See if one of you can host a regular campaign and if your friend can join (with all mods disabled). If that doesn't work then your issue may be a little bigger than my mod, but should be fixable by telling steam to "verify integrity of game cache" (just google it if you don't know how :)
Once you've both done that and you are both have completely up to date versions of L4D2 and you can both play a local campaign, then try plying Old School with no other mods installed.
Let me know if that works!