If a person does drink alcohol while taking keflex, do they have to restart the entire keflex regimen or can they just take one extra day of the antib?

Neither: Alcohol is toxic to living tissues (we humans happen to be a complex glob of living tissues), so avoid drinking too much. Also, an infected person doesn't need any additional, voluntary toxins in the body. However, alcohol does not have a direct relationship to how many Keflex capsules one needs to take. That would just depend on how well the infection is clearing up, and on the doctor's orders.