I have no idea why he would have done that. Fixing it isn't the easiest thing in the world, but it's doable. From reading up on this not too long ago for a 55 I have, trying to patch it isn't recommended as it'll typically just break again later. Each time it breaks, your tank's glass is stressed all at once and it's more likely to pop a seal each time. Note that everything below assumes your 90 is glass...I have NO experience with acrylic.
There are three ways you could go with this. No matter which one you choose, I recommend tearing it all the way down. It can be done without doing so, but it's much harder and you'll need some way to support the glass in a way that it's not flexed while you repair it anyway.
Method #1: order a new plastic rim for your tank and replace it. You'll need to remove the old one. Do this by getting one of the
paint multi-tool scrapers from Home Depot, Lowes, or Ace Hardware. Simply slide it up under the rim from both the front and back of the glass all the way around cutting the old silicone away from the glass. Once you've done that, you'll need to pry the old rim the rest of the way off the tank. Usually it's easier to do this if you cut it in at least one place with a dremel. Next, clean away any residual silicone and prepare the surface of the glass with some acetone. Finally, silicone the new rim into place and let it cure for about 3 days before refilling the tank.
Method #2: euro-brace the tank and screw the old rim. Have a 4" piece of glass cut to go along both the back and front panes of the tank. Silicone it in just below the existing rim so that it runs the entire length of the tank giving it extra support and keeping it from bowing out as much.
Method #3: Create a new, glass center brace. If you do this, you'll probably want to remove the old rim per the instructions above, but it's not completely necessary I guess. Cut 1/4" glass into three sections. The first will be the main support. It should be about 4" wide and as long as the inside dimension of your tank. Two of them will be siliconed to the front and back panels to act as supporting edges and additional silicone adherent surface area for the main brace. They should be 4" long and maybe 1/2" wide. Silicone them in a way that you can lay the main support along the 4" edge of the one on the front panel to the one on the back panel without the main support going above the rim of your tank. Once they're in, simply silicone the main support in place and let it cure.