If I knew the owner/operator well enough to know he'd appreciate it, I'd absolutely fix it. If I didn't, I'd leave a note over the
coin acceptor so no one loses money and whoever finds it knows there's a problem. I've actually done that several times, I drive all over Texas and test the washes I see.
I wouldn't fix anything at any wash without permission, purely for liability. You never know what kind of a-holes are out there. There's a guy who has bought up a bunch of washes east of me, I've never met him but I've heard many stories that are actually hard to believe, but I know and trust the people who told me. I still wouldn't load up a timer to destroy a pump for him, but if he had a chance to do me the way he's done the other guys I would.