You should probably spend a little time verifying that what she’s saying has zero merit, and then find a way to document it before it melts and turns into her story vs yours. From what you’ve described it sounds quite obvious her claim is bogus.
if what she’s saying turns out to have any truth to it at all, it could absolutely be grounds for litigation. If you create a situation on your property and it trespasses onto someone else’s and then causes damage, even if it was unintentional and it’s “how it’s always been done”, they can come after you for that.
had a situation last summer, we take care of 2ish miles of 10” water line that supplies irrigation and fire suppression water to a neighborhood made up of 5-10 acre estates. 3-4 times a year we shut it down to make repairs, and we follow a specific start up procedure which includes burping the line to let air escape as it fills back up, which means opening several fire hydrants and letting them run full bore for a minute or two. We open the same 4 hydrants everytime, have for years, we were taught to do it by the guy who installed the system. Well, in between our maintenance visits, one of the homeowners installed an underground vault “bug out” shelter in his horse pasture and put some expensive electronic well controls in it. He didn’t want any one to know about it so he buried the lid under a few inches of dirt. It also just so happened to be in the flow-path of one of the hydrants we have to burp. The run off filled the vault up and ruined the controls. He was mad at us for ruining his controls, I was mad at him because It was impossible for us to have any idea of his hidden shelter, but at the advice of the our lawyers our company and the hoa covered the cost to repair the vault rather than try and fight it in court. They said we didn’t have a case if it was our water, under our control, running onto his property. Long story, but maybe some food for thought.
id just wait for the next warm sunny day and go watch where the run off goes. Take some videos and document it.