Patching and Plastering - uneven drywall seam

Remodeling a fixer upper,ceiling needed a 4 ft by 5 ft piece installed from water damage.
three sides came out level and nice but one 5 ft side came out bad unlevel,recessed about 1/16 th to1/8 of an inch.My dad tried to plaster it sand it and called it good after painting it.
Is there any way to fix this to make it level on the one seam that he missed?
He is thinking of sanding it or grinding the existing drywall to level it up to the patch but I think you do not ruin something that is in place and good to TRY to make it level with a patch.
Please help before the mad grinder comes next monday.

Welcome to the forums!

He did tape the joints, right? Was the low spot on the factory beveled edge?

While it is ok to to sand the joint compound, you never want to sand thru the paper face of the drywall!! It is better to float the j/c out further to hide the joint.

Marksr,Thank you for your reply to my drywall problem.The edges in question are both cut sides not factory edges.They have been taped,sanded,taped and painted.
Do I have to cut the whole line of 5 ft. tape and start over or can I just remove the uneven section and then float it?
Do you feel that fiber tape is better to use or paper tape?
Is floating difficult to perform to make level?

Paper tape is always better than the 'sticky' [fiber] tape.

Unless the mud was built up way too much under and over the seam, the best fix is to widen the finished joint. Basically each coat of mud will go out further [wider] than the coat before. It isn't uncommon to have a butt joint 2' wide. The final coats of mud should be applied with a 10" or 12" knife. Making the joint wider may not make the job completely level but it should make it where it isn't visable - which is really all you can expect - an illusion of a perfect job :D

Ceiling. Nobody is going to go up a ladder and hold a straight edge up there. Just do as Marksr says.