If the good blade can be matched with a new blade from your inventory, that's the best way to go.
If the good blade can be matched with a new blade from your inventory with just a bit of tape to get the balance, there's nothing wrong with that.
If you can't get a good balance, replace both blades and SAVE THE GOOD ONE YOU TOOK OFF. You'll eventually get another set of blades that may match that one and you sure don't want to toss good blades.

I've never found a pair of blades that I couldn't match with a strip of packing tape about 3/8" - 1/2" wide and a couple of inches long. Always put the tape on the underside and smooth it out completely.

CA the cracks in blades so they don't continue to fail.
If you glue broken out chunks of blade, it throws out the balance, so check that. Sand the repair smooth after it dries or you will mess up the aerodynamics of the blade.
If you get TBE or ROTATION out of the heli with a repaired blade, dump the blade.

I've gone nuts trying to retune the heli to save a blade. It's a buck. Get over it.
Soloboss