Way back in the early 80's I let a friend take my bike up a muddy lane near his house. He putted a hundred yards or so, up and back without incident, then watched me flying up and down flat out.
"If I'd known you were going to ride it like that, I'd have gone faster. Can I have another ride?"
The results were predictable and extremely painful for him, with some deep bruises and several patches of mud-impregnated gravel-rash.
This person is still a good friend, spent several years as a master instructor after he finally got his own bike, and can ride any of my bikes any time.
Another time it was the very experienced instructor who had left his keys at home so would I mind letting him run home and get them. He came back so quickly, with the engine ticking like mad as he shut it down, he had to have been thrashing the heck out of it. He then proceeded to moan about the condition of the front tire. He will NEVER use one of my bikes again.
These days, I'll only lend a bike to someone I know well enough to be sure they can ride well, are going to be responsible with my bike and would fix/replace it if they caused any damage. That's a pretty short list of people - all of whom would lend me their bikes too.
Test rides are another thing altogether. Nobody gets test rides on a bike I am considering selling until I've got the cash in my hand and even then not unless they can convince me they are not going to dump the bike before they've left my driveway (been there, done that!) If I lose a sale that way it's not a big deal to me. The last couple of bikes I sold, the buyers did not get to test ride them at all.