a. has an easy literal meaning. "I am able to assist him in the process of becoming rich".
b. Is ambiguous between the literal meaning "I am not able to ...", and the idiom "I can't help X".
The idiom means "You are X, and there is nothing I can do about that.", and so the idiom would mean "He has become rich, and there would have been nothing I could have done to have prevented it." Or "It is my fault that he got rich."
The idiomatic sense would be used somewhat ironically. For example, rich people are sometimes thought to be arrogant, so you might say "I can't help your brother getting rich, but that doesn't mean he has to treat me like dirt."
But without context it is ambiguous, I would probably assume the literal meaning "I am unable to help..."