Although in principle, I think that people should carry their own pens, and it's not my business to rectify their shortsightedness, it's not a perfect world, and it's just barely possible that I could get caught short myself some day. So I generally have a ballpoint available to lend, and will usually lend it. If I'm out of ballpoints, they aren't getting one of my fountain pens. Maybe they'll remember to bring their own pen next time.
If it's someone who does have his own pen, but is curious about my fountain pens, and would like to learn more, I actually have no problem risking a reasonably good fountain pen, like my Lamy Safari, Pilot FP 78G, or Kaweco Sport. The thing is, that I almost never carry those with me (or keep them inked any more). I don't care to risk the nicer pens that I actually do carry around, and which suit me perfectly, so I've asked people to wait until the next time I see them, and I'll lend them a pen and some ink. I even tried to give away my Safari, but the friend who tried it didn't like it, and gave it back. He hadn't ruined the nib, though.
"I am not a wise man; and besides am a mortal of so little consequence in the world, it is not much matter what I do; so I seldom fret or fume at all about it."
Laurence Sterne: "Tristram Shandy"