Unless it's health-related or because of some personal turmoil, I don't see this sounding good for the program.
His departure could be for many reasons but I'd suspect that an other assistant has been given a little more security then him and he probably did not like it. Instead of waiting it out he's taking off. If that is the reason I would not blame him. It does not make it right or wrong, it just is! How many guys here have not liked their jobs or situation and had the ability to just cut and leave?
I'm just guessing here guys! Also there could have been some personal issue that none of us know. Regardless, I wish him luck.