No, because she's not even a diver. The "good diving" thing is just for me.
I'm just back from 3 weeks diving Japan, so a couple ideas. Keeping a non-diver happy does complicate things a little, as does the fact that the best diving is generally in more remote/isolated areas.
Being based in Okinawa with you boat diving local sites and/or the Kerama Isl should meet all those criteria. Zamami looked beautiful for non-divers, but I don't think you could be based there as it might be classified as isolated -- though it's only an ~80 min fast ferry to Okinawa for hospitals, etc. There's lots of shore diving on Okinawa too. I did not see any area that looked interesting for non-divers where we were in Yomitan or Chatan (both good Okinawa bases from a dive perspective) and we drove around quite a bit, but maybe we missed them. You could definitely stay in an Onna beach resort. You could also stay in more of a touristy area and commute a bit further for diving.
We loved the diving in Hachijo-jima and our dive op was spectacular, but without a group of 3 people you'd likely only have shore diving options and depending on your experience/preferences you may not love the shore diving as much as we did. Only a 50 min flight from Tokyo, but on Hachijo itself not sure whether there's a serious hospital, etc. Between cooler water temps, very little for a non-diver to do and very limited English outside of the dive op I do not think it's the best bet for your trip.
Logistics for non-Japanese speakers for Izu peninsula diving made booking extremely challenging, especially for weekdays, but again some very interesting endemics and small animals there (we did not do any boat dives because of the shorter time limits). You'd have to look into nice places for a non-diver to stay (we stayed very locally in
minshuku) and whether your friend could find enough to do. I know there are nice places with
onsen in the region - might be possible if you are willing to commute to and from the dive sites.
I should mention that the diving we did in these areas of Japan was nothing like diving in the more famous areas in Indonesia, the Philippines, Thailand, Sipadan etc. If you're looking for more of a 'dive magazine' type experience, Japan may not provide that for you. Though if you're going to hit the rest of SE Asia on your own later anyway, having an organization help with the bill for Japan's higher accommodation and meal costs would be clever.
We loved Japan and hope to return. Bali's great also though.
