I’ve had this issue multiple times in different rentals and the easiest, the most obvious, and sometimes the only, way to figure out what’s going is to open up the ceiling where it’s leaked. Anyone can do this with simple tools assuming it’s a drywall ceiling.
So you tell them to open up their ceiling, then they peak around and the source of the leak will become obvious. If not then they coordinate with your tenants to run water to source the leak. IMO, the burden of proof is on them to investigate further and prove it’s coming from your unit. Obviously the chances are that it is your responsibility, but what if it’s a sprinkler line or drain line owned by the condo building? We won’t know until they just open the damn ceiling.
I would have been less nice and just reply “hey, take 2 minutes, open your ceiling and stick your head up there. Then send me some photos. No one is going to tear apart a bathroom as that’s tracing a leak backwards.” Sounds like your tenants neighbor is an idiot honestly. Email looks fine.