What selenak said. The only sites of possible Fritzian interest I can think of that are on that side of the country, and I'm talking like a 2-3 hour drive south, are:

1) a very minor museum on the site of the spot where 18-yo Fritz tried to escape from his abusive father but was caught, then sent off to (modern-day) Poland for imprisonment and traumatic brainwashing rehabilitation (and yes, as selenak says, he was on his way to somewhere else when this happened),

2) Hohenzollern castle, waaay in the south, which Fritz himself didn't live at or even visit, but the current (19th century) castle has a museum that has some Fritz-related stuff (and also other things of possible interest to Americans, such as, Wikipedia tells me, a letter from George Washington to von Steuben). Also Fritz (and his abusive father Friedrich Wilhelm I) were buried there from 1952-1991. They're currently buried in Potsdam.

I haven't been to either site, but I can tell you that while (2) might be worth a trip if your friend has the time and interest, (1) is only even remotely of interest to people who are absolutely obsessed with the escape attempt, so basically just me. ;) (cahn and selenak may remember (1) as the site with the pre-1945 plaque saying "Here on August 4/5 Frederick the Great was preserved for the Fatherland," i.e. "Yay child abuse leading to conquest!")

In terms of things Fritz did in the Rhineland vicinity, he visited Strasbourg once with a forged passport and spent the night in jail (no really!), and about an hour away, he saw his first sort-of battle as a young man, at a very lackluster siege in a war that no one remembers any more, but there are no "Fritz was here" tourism sites there. (Though Strasbourg is interesting in its own right.)

But if you, your friend, or anyone you know should ever end up in the Berlin area, we can give you the Frederician itinerary to beat all itineraries! :D