My girlfriend has a box turtle and it's developed a sore on its cheek, at the corner of its mouth, recently.

It's been really active, which I guess is a result of the changing season, and I kinda think it scratched itself.

Should we consult a vet?

Pics of both to help decide. Reptiles have pretty good immune systems. I'd smear a little Polysporin on it twice a day for a few days and see if that clears it up. Also look for anything sharp in its enclosure that it could be scraping his face on.

ETA: How long has it been in captivity? All my box turtles are wild so my mind didn't go to the problems of a "pet" turtle. Diet, humidity, access to sunlight, proper humidity and even a lack of hibernation can lead to problems.