I'm not aware of any case where that has happened in a civil proceeding. There have been many cases where people who have approached the RCMP with respect to people they knew or who were in the program and asked for service by the police on that person who has been affected. They have been able to commence civil actions without any prejudice to the fact that the person was in the witness protection program.
On June 7th, 2007. See this statement in context.