Got my mum a (renewed) 6S plus one or two years ago, looking back it was a good choice - instead of getting her / spending a lot more on a more recent model, as well as getting her a bigger ("plus") model instead of a smaller one - her eyes are still fine but she really enjoyed having bigger screen / larger keyboard and longer battery life).
So my suggestion now would be a refurbished / renewed 7 or 8 plus.
Got my mum a (renewed) 6S plus one or two years ago, looking back it was a good choice - instead of getting her / spending a lot more on a more recent model, as well as getting her a bigger ("plus") model instead of a smaller one - her eyes are still fine but she really enjoyed having bigger screen / larger keyboard and longer battery life).
So my suggestion now would be a refurbished / renewed 7 or 8 plus.