Faith's friend, who is a 16 year old girl, named grace met up with her one day. the next day, faith saw her in the store and went up to her. her friend didn't know who she was, and was completely different. she said her name was john, and that she was a 22 year old man. why did grace do this?

Grace has split personality. 

Grace has what is called Dissociative identity disorder, or multiple personality syndrome. When you have this condition, a person can change from one person to another (in their mind), and they can't remember anything about the other personality. 

Hope this helps! (Can you please mark brainliest? Thanks!)