Not sure if this will help but there is a property in the character file called “expansion-character?”. My characters all have a value of 3, probably because I have both expansion packs and have completed the game from base-AoM-FG. If I create a new character that has not started yet this value is 1, even though I have all expansions plus base active.
Look at your character value and get your friend to do the same. You can change that value to whatever they have and see if that works. Make sure you back up all your characters before you make any changes so you can recover if this turns out to do something bad or undesired.