My friend developed brittle, cracking nails. What vitamin is she lacking?

I have a friend who has developed brittle, cracking nails. Could you tell me what vitamin she could be lacking? Thanks in advance.
Nails are made of protein so unless she is lacking protein in the foods she eats, she is probably not lacking any vitamin. Nails suffer from the physical trauma we put them through on a daily basis. Dishwashing and cleaning products often dry out nails. She can buy a nail moisturizer or use any moisturizing lotion regularly to keep her nails softer.
Your friend should discuss her brittle, cracked nails with her doctor on her next visit.