I looked over the contract, and it just lists a plumbing budget of $8000 without specifics. But it also says that his bid is for no more than $83,300 for both phases of the project and that if for any reason he finds he needs to go over that amount he must submit a change order to us.
According to your above statement you could go after him for not submitting a change order.
This is business! There is no room for being nice. If the GC DID NOT SUBMIT a change order than ANY changes He did are on his dime.
Go after Him DON'T DO HIS WORK FOR HIM!
The reason the other quote was higher was He saw these things coming. Unfortunately your chosen GC did not. Let this be a life lesson for Him. Guys like your GC have to learn the hard way. That means money out of His pocket.
It's not your fault that your GC wanted nothing in writing. BURN HIM!!! Make Him pay!!!! There are too many guys like this in our industry that RUIN honest GC's reputations.