Intrigued by the problem, I asked MyRoute to give me a route between Ouistreham and Limoges ie. mirroring exactly the simple A to B route, created by the OP’s friend.
It spat out exactly the same route as shown in post #3, inch for inch. Post #3,was created in My`Route from the file in post #1.
I think:
A. The file in post #1, does not contain a route or even a track at all. I think it just contains a start point (Ouistreham) and an end point, Limoges. In short, A and B, but nothing concrete in between.
B. This would explain why Pocket Earth displayed a straight line; there is no data (other than points A and B) for the app to latch onto.
C. I can also create a straight line in MyRoute by asking it to import a track version of the data in the file in post #1. It too, then displays a straight line, running between A and B, Ouistreham and Limoges.
View attachment 308840
D. This would very probably explain the straight lines shown in various different GPS devices. They have done their best, with nothing but points A and B.
I am happy to be proved wrong but I smell a strong pong of human error, I’m afraid. If so, better to realise now, than when rolling off the ferry in Ouistreham. Actually, thinking about it, that would be OK. Just do what I think the OP’s mate has done…. Ask the dumb (but very clever) GPS device to “Take me to Limoges, shortest distance”. It will do it, without complaint.
What to do now?
1. Maybe create a proper route A to B, taking the roads you want to take, rather than one being spat out by a dumb (clever) piece of software.
2. Perhaps wonder whether you need one route that long. If you do, great. If you don’t, break it up.
3. Make sure the file you share between you all, does contain some meaningful data, or at least something beyond (and between) A and B.
4. Check it all and then check it again, once it is installed and displayed on your assorted GPS devices, preferably well before you leave home.
5. Enjoy your holiday.
6. Buy a map; you may yet need it.