Me and ralath just tested it out again...
He's got 2 mins left of a GM buff with like 1.9k HP....
I went and buff him... he said timer restarted... but his HP didnt go down...
2 mins later, GM buff ran out he went back to his normal 600HP...
vivi's almost killed him MWHWAHHAHA
(i buffed him afterwards and he's on 1.1k HP)