Audra and Geoffrey both made excellent points. If the gymnast's back is only hurting when they do back walkovers, most likely it is due to poor technique like Audra said. If it is hurting more frequently than only back walkovers there could be something wrong...not to scare you. I was a gymnast who, until college, had stronger chest muscles so i have slightly rounded shoulders. I also have a fractured back that is never going to heal.
I competed for four years on a back that was hurting all the time. My junior year of high school i decided to go ahead and get it checked out--turned out that L5 (lower back) was fractured. I went on to compete my senior year and make it on West VA University's gym team--but in Sept of my freshman year my back was hurting so bad i couldn't even sleep at night. It turned out that my back was fractured yet again and this time--it didn't heal, so I was forced to retire.
Back problems are common for gymnasts, if they do their ab strength correctly and they condition all around their body, I feel, that they will be less likely to have a back problem. My back problem was maining from over use. Don't be alarmed if your gymnast comes home complaining about their back, check with a physical therapist for some rehab exercises for your gymnast to do and that will strengthen the correct muscles for supporting the back. Not all back pains are horrible--sometimes they're just sore and the gymnasts do not know how to tell the difference.