If you are not a native English speaker, please note that the English phrase is incorrect. He passed well would be better.
Reported (again) April 2018
Why is this vizuri and not mzuri?
The English should be "He has passed well." Reported.
Because it's an adverb in this sentence.
As it's been said this is an adverb, but to be specific when using adverbs we treat the verb as if it is a vi- class noun. So anytime you describe an action the adjective leads with vi-