The Fai in the picture looks similar to the Fai in the prison, so it could be a picture of him after Ashura took him out. Also, Fai ages slower so it's hard to tell how much of an age difference there is between the two pictures, but the character guide picture could well have taken place after Fai was removed from prison, I think.
While I certainly think your idea has merit, and I'm not 100% sure that Ashura was *actually* the one to take him out, I don't think that picture can really prove anything besides the fact that Fai knew Ashura when he was younger, whether it be before he was tossed in the prison or after.
Though I don't know if I see Fai and Ashura having a parental/son relationship, personally. While it may not be romantic, it doesn't really seem to have that dynamic to me.