What's the problem
The picture shows some dark spotting on tomato leaves. Is it only on the lower leaves? You indicated it's only on one plant right now. It looks to me like the leaves were exposed to some sort of temperature change or chemical exposure. Any lawn chemicals used nearby? Car exhaust? Cleaning solutions? Paint?
This symptom doesn't look like a plant disease or insect problem. I would leave it for now, and evaluate the plant in a few days to see if the new growth looks okay. If not, you could take a sample in to the Cooperative Extension offices on 16483 County Seat Hwy, Georgetown, DE. 302-856-7303, so we could look under the microscope.
Thank you for contacting Cooperative Extension,