I have heard of this happening one time. He had to remove his tats and get new ones and he had to have someone vouch for him. If he returns to prison he would be targeted due to being a sureno, but would have an added insentive for being hit due to his drop-out status. Best bet would be to move to a non gang area, change his appearance, clothing, remove all tats, friends and types of people he normally would hang with, end the gangster lifestyle, and stop all contact with prior friends or family that are in or associated with gangs and hope that no gang members see him and recognise him.
Additionally, I have seen brothers where one was a Sureno and the other a Norteno. Usually, they will have a truce while in the family home, but outside of the home all bets are off.
As the prior answer, best bet is to never get involved with gangs.