Could you please advice; what would be a very userfriendly and safe Android app to be used by the less tech savvy
to buy some BTC and keep them in their own wallet on their phone?
While it's best to always keep your bitcoins safe by keeping them in cold storage ("paper" wallets), from time to time it's necessary to temporarily sweep some of them into an electronic device to transfer them.
For this I recommend Mycelium. It is the mobile wallet application used by all of the OTC dealers I deal with.
It's very easy to use and is as secure as any mobile wallet. While it had some issues with iPhone users at one time, those issues have since been resolved. As an Android user, you should have no problems.
It just plain works. I haven't used anything else for years.
same here, used mycelium on android for years. i buy at exchange and transfer to wallets under my control (hardware usually). mycelium uses seeds so can be backed up to paper. its pretty easy to use.