You don't have to have been charged to have 5th ammendment rights or be able to exersize them. He could have pled the 5th and not said anything whether or not

You don't have to have been charged to have 5th ammendment rights or be able to exersize them. He could have pled the 5th and not said anything whether or not they already charged him, it doesn't change that. (If they grant you immunity , so they can't charge you even in the future, that's the only thing that gets you out of being able to plead the fifth).