No worries about fish safety. From what I remember the can did say 10yr. warranty, however, I believe it was for use on concrete, not directly on plywood. MAY still work just as well, but it's not been proven(to my knowledge), which is why I'm so interested to follow your tank. I did a plywood 180G sealed w/drylok a few years ago. I had it running for a year w/absolutely NO ISSUES. I ultimately had to sell it due to a move, so I never got the chance to find out it's true longevity. I do know TheFishGuy did the same thing on a few sumps and smaller tanks, but said they started failing around the two year mark. How long has your friend had his 240G up???
Forgot to add that, I'm currently using a piece of 3/4" plywood sealed in drylok as a replacement panel for a 75G w/a cracked side. Planning on using the same method for a 55G I just picked up...