Use the other tree as a clue to move forward but do not assume the information is correct.
For example, in my husband's family there is a sister to his grandfather that even my father-in-law had never heard of. I found her on the ship's manifest when they immigrated, so I'm sure of her existence. She was a teen and disappeared before the first US Census the family was in. I have found no trace of her. My guess is she married and changed her name.
She appears on several trees on Ancestry (with her parents listed as my husband's grandparents). I've written everyone who has her on their tree and only a couple have bothered to write back. They say they copied this info from someone else and do not have a source.
No way am I going to consider this information as legit until I can confirm it. But I do have a potential married name, children's names, husband's name, etc. This means I can do more searches that might be fruitful. For example, I might find one of her children's marriage or death certificates that give the mother's maiden name (it's a very unusual name).
So far, I haven't had any luck, but the other trees are giving me the clues I need to break this brick wall. I just need to research it some more.
Good luck with yours. Be sure to write the other tree owner and ask if they have a source.