Oh gosh, I studied it from a textbook in Polish, and now that I'm trying to relearn the language i'm heavily relying on duo, too. I do have my textbooks, but that's not much help for your dad

I also use memrise, but that's not as reliable because there's no official Hindi course made by them. I can't think of any source in English, i guess you'd just have to find some textbooks. I'm sure you have more than we had in Polish sources. I would offer chatting with him but that might not be the best idea for him

sorry i'm so not helpful

But duo is pretty good for a start, I recommend reading the comments, they can be very informative.
What peaked his interest in Hindi of all the languages?
Edit: I forgot about the app called 50 languages, there's Hindi there and it's pretty neat if I recall. I have no idea why I don't use it anymore, I should
