I have seen flat spots on several pens (Parker 51 and EstebrooK 2xxx and 9xxx nibs).
On some the flat spot was REALLY FLAT, almost like they wrote on a sharpening stone. They must have had a HEAVY hand.
I was able to reprofile the tips to round them again, so they are decent writers. Luckily I had enough tip material left to do that. And I have a few more to reprofile.
As others have said, there are other reasons to not let someone use your fountain pen.
BUT, that might be a good reason to tell them why they can't use your fountain pen. Like the State Farm commercial, its on the internet, and they can't put anything on the internet that isn't true
What I would be afraid of (besides dropping the pen) is someone writing with it like a ball pen, pressing HARD and springing the nib.
I got my wife a manifold nib for her Esterbrook for that exact reason. The manifold nib will handle her pressing down, since it was made to do carbon copies.