There are a few cases in Tokyo, Kyoto, and Osaka, where I'll be. The other two places I'm going seem less affected but also only make up three days of my trip.
I can't imagine a scenario where I don't go, unless there's a travel restriction and I can't, but it is a little nerve-racking.
The CDC currently has Japan at a level 2 advisory. Which means they think travel is fine with necessary precautions. You can probably show your friend that page.