This is a known issue. It should help if you reload the save, possibly multiple times; your friend shouldn't have to quite the game and restart. It may be more reliable to join in an older save, or new game, and then load the latest save.
A temporary attribute boost, such as from the Man At Arms skill Encourage, may also reset the character stats properly on the client system.
This has rarely happened during testing internally, but it isn't reproducible, so a fix remains elusive.
