Have the clerk put the phone's display into Zoomed mode. It is what I use to help my older eyes see the screen more easily.
It's not a text size setting. Text size settings can be ignored by many apps. Zoomed mode is for the whole phone at all times in every app.
Zoomed mode makes the whole screen zoomed at all times and really helps make it easy to see. The availability of zoomed mode is one of many reasons to get an iphone. See examples below.
Try the iphone 5s, 6 and 6+ in zoomed mode. Find which one is easiest to see and hold for you. I am guessing the 6+ in zoomed mode will be the best. For some the 6 is a good balance between size to hold and see.
Cheers.
Example zoomed:
Not zoomed:
