My Wife's Tongue Is Red And Feels A Bit Sensitive.

My wife's tongue is red and feels a bit sensitive. This has been going on for the past 2 months. She went to see her dentist and was told that she may be lacking a certain vitamin (C2 or C12?). Her dentist told her she could go to her doctor and get a blood test. Her dentist also said to go to WebMD and maybe find out some fruits and vegetables to eat instead of vitamins. Thank you.