Here we go..... someone wants to learn how to use a Sextant because they want to do it for FUN. The Mark 15 or 25 from Davis is perfectly fine for that. I have a 15 myself. Recommending apps for your phone, talking about accuracy at sea and what not is not helpful as that is not what the person wants to do. Some people just want to learn how to do it for themselves. Really, there are people that do that. It's called educating yourself and learning something new and interesting to yourself. As was stated a couple of times by the OP, their friend has an interest and just wants to learn how it is done.
One other thing, I don't think mine has been out of the box more then a handful of times and none of those times were to do any sights....... So, spending a little is prudent in the beginning. Would I like an Astra IIIB? Sure but, expensive dust collector in my house.