It's called at-will employment. Actually, I think he may have a legal case to be paid the original offer from the time he started working up until he got notic

It's called at-will employment. Actually, I think he may have a legal case to be paid the original offer from the time he started working up until he got notice of the lower pay (a week or two tops) but that's a very small amount of money for a very big deal court case. His time would be better spent getting out, which he did.