Redsy,
I've seen many such installations where I am told that the Landscaper had installed the lighting. Typical installations have minimal cover (if any) and buried splices with wirenuts and tape. It is inevitable that something will fail or be damaged, hopefully it's not a person.
You are right that GFIs' fail, especially ones installed outside. I've seen a Pool Light with a broken lens (Bulb & Socket in water) that did not trip the GFCI that was 'protecting' it.
As far as over-reacting goes, No one can tell where or when a Serious Accident will occur, but it is always best to try and reduce the odds if you can. It sounds like you did your part in notifying them of a possible problem, it's their Gamble from here on out.
Bill
