Agree with the above. I'd add to ask what questions they have. (I've found that few newbies have questions before the game, but most do at half time.) And really keep in mind not overwhelming--too many mentors give too many suggestions, which means they can't be processed. One is pretty conservative, but for some that is all they are going to be interested in hearing. But I'd put three as the outside number. It can be challenging for those of us who know so much to rein ourselves in, but less really is more in terms of retention of the information (and of enjoying the experience and being open to future feedback).
(And I laughed at the repetition of "louder whistle," as it is absolutely true, especially for youth refs. When I teach our intro ref class and there are youth in the class, I tell them to go home and blow the whistle in their room to practice--and they aren't blowing it loud enough until their mom yells at them to knock it off.)