Given other users are able to run the applet on the same machine, I suspect the applet and Formativ are correctly installed on the PC. It sounds like it may be a problem with the users account itself. You may wish to try performing maintenance on the users account.
If performing maintenance on the account does not help, I would suggest you may have insert some MsgBox() or similiar lines in your applet to determine the path of execution when the user tries to forward a message. This should give you a better idea of the problem, and how to fix it for this particular user.