Hmmmmm OP, you could play at him at his own game there.
Two things. If you didn't mention what your working was day was that could be 9 to 4 , but if more realistically you are like most of the trades I have used start is normally 8/8.30 and finish at 4/4.30.
And in my book if they finish early and the job done is good - good luck to them, as quite often on other jobs delays can crop up (i.e. material availiability not checked beforehand ) which can mean they take longer than expected, but that is their misfortune. Unless it is something byond their reasonable control and is negotiated while the the job is in progress.
Hope you do get the money you quoted, but if he continues to be a pig, not worth losing sleep over and just make sure you don't help him again and your friend knows why that will be the case