I'm afraid I'd feel I had to tell them. Then it's up to them, but I hope they'd at least look again, to check out what they have, and consider if they may be mistaken.
I hope someone will tell me if I ever get climbing out along a branch that's not really attached to my tree, before I fall off it with egg all over my face.
I nearly did once, researched what seemed like the right parentage ... but somehow it didn't quite seem to "fit" properly, so I went right back and started again ... and found I'd slid over onto a cousin, with the same names of children, almost the same ages, and in the same general area! I could so easily have adopted this branch as the right ones! Even the job fitted.
At present I'm going cross-eyed trying to search into the back-story of a chap who spent 2 censuses with a direct ancestor, claiming her as his wife (I don't think they married at all!) before she goes off in the next census and claims to be married to another chap (and I know he was married to someone else!) but I can't find him before or after, and he claims two different dates of birth, nearly ten years apart! I've found someone who so nearly fits .... but no proof, so can't get there.
Isn't it frustrating? But isn't it satisfying when we do get an awkward so-and-so of an ancestor finally pinned?