I can see why this sentence can be confusing. Let's address "in any event" first. The phrase "in any event" generally means "no matter what" or "regardless." In this case specifically, it means "no matter what he decides." "I think he'll agree to do it, but no matter what he decides, all he can say is 'no.'" The other part of this sentence that can be confusing is the way the last part is worded. The way this is worded, "all he can say is 'no,'" literally means the only response he can give is "no." You are correct that "agree to do it" contradicts the literal meaning of "all he can say is 'no.'" While this wording choice is unclear, a native speaker would likely understand that the speaker means, "the worst he can say is 'no.'" So now the sentence would look like this: I think he'll agree to do it, but no matter what he decides, the worst he can do is say 'no.' I hope this helps.

In any event = no matter what the circumstances are. So, the speaker here is basically saying: the worst that can happen the worst case scenario) is that he will respond with a "no" (even though the expected response is "yes'). In other words, it's unlikely that any truly negative outcome will result from making the request or asking the question -- "all he can say is (the worst that can happen is that he can say) NO." Does that make it any more clear?