I think he might wish a bit of advice

Temporal RP have one good point which also can make them a bear. As long as you only travel backwards you can base an adventure on an important point in history using real history as the backdrop but then comes the hard part "How to involve the crew?"

They could play minor but important points in shaping history, a favorite Time Travel stunt is to make them have to chose from their morals which if observed cause history to shift in a manor that corrupts their future or place their morals on hold to do the one thing that will set things on the proper course.

Another is the Alternate Timeline visit which is fun in some ways but tough to keep interesting

Oh and a "Just Observe, Don't Touch" is boring and could be done in books and media; yipee! Let's go to the Library