The disclaimer at the airport is essentially to cover the airport, not to cover drivers from each other. You see the same in retail car parks, but I know of a case in a retailers premises recently where minor damage was done to a customer's car without the culprit realising it, and the gardai arrived at the culprit's door within a week having reviewed the video evidence from the retailer.
As long as you are not implying the airport are at fault, and you are only looking for evidence of who the other culprit is, they may view the request sympathetically.
One possible problem though ... in the retailer's case, the owner of damaged car was away from the car for (max) a half hour, so not much tape surfing to be done there. There is potentially 1 week of video tape to review to get to the bottom of your relative's case, so it may be a lost cause, given the effort required.