In this type of sentence, "could" doesn't mean "was able to" like in this sentence:
You could see Times Square from the window of his office.
Instead, "could" is used as a suggestion. It means "He should say hi."
The phrase "(someone) could at least (do something)" is used when you think someone is acting totally wrongly. In the example above, the speaker actually wants the director to be friendly and start a conversation if they see each other. But even if he doesn't act that friendly, she thinks that he should at least say "hi". So saying that someone "could at least ___" is a way to complain and tell what you think someone should do as the basic, minimum effort.
Other versions of this are:
He could say "hi" at the very least.
At least he could say "hi".