My first thought is that the guard might have been paid to let this happen, and he just has a lame excuse.
I know a bit about prison security, and I doubt they have our standards of humane treatment in The Phillipines. At any rate, contrary to what movies might show there isn't just one set of keys that will deal with all doors in any prison that relies heavily on lockdowns. The idea being so that this specifically can't happen, and one guy losing his keys or missing a door won't let them get out, they need multiple sets carried by people who aren't in the same area all the time to unlock multiple sets of doors.
If this was a minimum security type set up, where the prisoners are allowed more or less free reign within the prison grounds, and people on work details can flee (light sentences making the potential consequences not worthwhile), I could see it more easily however if the guy was distracted, but then again these guys could probably have walked away at any time in that case and the game probably had little to do with it in any genuine sense.
At any rate if they can trace it back to one guy, irregardless of the circumstances, I'd be looking into him being a willing accomplice.