Screen goes off when the back is tapped

I have an ipad2 wifi, works fine except if you tap it in certain places the screen goes off and you have to press the lock button to wake it up again.

no visible damage on the chassis but asking the owner (he's asked me to fix it) says it goes off if you tap near the ribbon cable area (left hand side bottom half) mostly.

i suspect a faulty ribbon cable to lcd but also think it could be a faulty lock button shorting?

any ideas?

thanks

dyfed