Your friend may have had a download of iOS 8.3 downloaded from years ago when it was new so it may have been pre signed by apples servers then and hence why he may have been allowed to update. As for iOS 10 being slow, I don't think it's really a huge issue here's a article explaining performance. https://arstechnica.com/apple/2016/09/good-news-ios-10-runs-pretty-well-on-the-iphone-5-and-5c/. If you've been keeping tabs on iOS 10.3 this will also further increase overall stability/performance. Also it will introduce a new file system so waiting till next week for that to release and then doing your full restore/set up as new would be a ideal path to take. I can tell you first hand iOS 10 on iPad 4 isn't bad at all, yah it's alittle slower loading things but it's a negligible difference and the app support/new features greatly out weight the performance hiccups here or there. iOS 8 by contrast was rather radical with new features/api so even on newer devices it was rather buggy and performance could vary greatly were as iOS 10 is pretty light on new features while further polishing overall stability of iOS. I'd say iOS 10 is iOS 9 performance wise if not slightly smoother as iOS 9 had its own few hiccups performance wise. On my Air 2 iOS 10 runs noticeably smoother than iOS 9 seemed too. I'm really excited for iOS 10.3 for even more stability/optimizations.
Didn't know that, thanks for the information as well!