Had to replace sub floors in mobile home. Installer said underlayment is not needed. New floors are moving up and down and creating problems. He also used 1/2 plywood and there was some past repairs done with 3/4 . He did not remove all the 3/4 and put the 1/2 next to it.
1/2" is not suitable for floor use by itself, and putting 1/2" next to 3/4" (creating a 1/4" bump) is clearly not acceptable work. Unlikely that you'll manage to get the person who did that in the first place to do it correctly, but you can certainly give them bad word of mouth around town or electronically so others don't get burned.