5970G is one of the nicest Patek I have ever seen. It wears beautifully on the wrist.
Though it is 40mm, it wears much smaller, very similar to Lange Datograph.
Its dial is totally mesmerizing.
I currently wear it everyday to work 
If not worn for a period of time, there's a quick setting button which will help you set the calendar very easily.
This function is not available in 5 Patek 5059 or 5050.
The current selling price for this piece at reputable pre owned dealers is around $110k, so your friend will not lose much, if any.
But , seriously, this will be a watch to keep for a long time and your friend will be happy.
It is that nice a watch.
Jmho.
*CL