I would definitely take the kitty to the vet. To me it's the same as finding an injured animal along side the road. You would take it to the vet. Only in this case, you are pretty sure where the kitty lives. I'd pretend ignorance and get the kitty to the vet. You can always explain to the owner later your "confusion" but concern over the kitty.
I absolutely disagree with the idea of rehoming it based upon YOUR assessment (or the assessment of anyone) as to whether they are taking adequate care (as you see it) of the cat.
A neighbor decided that Kuhio had been abandoned or starved because of her appearance. Kuhio was 17 and on medication for a hyperthyroid. The neighbor called Animal Control who put my baby to sleep before we could rescue her. (They only keep cats for 3 days. My husband and I were out of town and our housesitter didn't even bother to look for her). My neighbor thought she was doing the right thing. She didn't recognize Kuhio because she hadn't seen her since she got sick. If she had just fed her (if she thought she was starving) and let her go, she would have come home.
I see nothing wrong with getting the kitty some medical attention, but don't assume you know the whole story and that abuse and neglect are involved.