This worked for me:
Open Google Maps
Centre on Santiago de Compostela
Make sure that the search bar at the top is clear
Touch
Saved at the bottom of the screen
Then scroll right to the bottom of all the content you see
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Now touch the blue Maps icon just above the bottom menu, see screenshot above
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A new screen will appear (see above). Your friends map should be at the very top (unless you have done other stuff since viewing it), if not scroll down until you see it. It has the same name as the top entry above.
Touch that map entry
You will see the map on a screen with (probably) a blue line.
At the bottom of this screen touch
view map legend
Scroll through the legend until you see an item that does have a tick in its tick box (which one probably doesn't matter but I ticked all of them).
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Once you have finished ticking items, use the back arrow to exit this screen. Do NOT click on
Close
You will be back looking at the map and now the line or part of it will probably be yellow (if you ticked everything).
If you now place your phone in airplane mode the map will stick around
It is very long winded.
Once you have done other things with G Maps this map will disappear. To get it back, go to
Saved _
Maps and touch the name again. It may not always be at the top. You probably need network coverage when you do this so remember to do it when you have WiFi or other coverage.
This is a somewhat dependent on network coverage and so if you want guaranteed access to this map when offline then you probably need to load it into a different mapping tool.