Reviewers should be objective.
It's clear that you've formed an opinion about the code in question before you've even reviewed it, and it sounds like you and the fixer have staked out positions. If that's so, then you're going to have a difficult time appearing objective, and an even more difficult time being objective. None of that helps the process, and it may be that the best, most objective thing you can do is to bow out on the grounds that you're too close to the issue.
Consider a team approach.
If it's not possible to remove yourself, perhaps you can have several other engineers review the code at the same time. Either they'll agree with you that the code should be rejected or they won't. If they agree with you, then it will no longer be just you vs. the fixer, and you'll be able to make a stronger case that the team looked at the fix objectively and decided against accepting it. On the other hand, if they decide to accept the fix then that too will be a team decision. It should go without saying that you should participate with as open a mind as you can manage, and that you shouldn't try to influence the other team members' opinions by anything other than rational discussion. Important: if there's a bad outcome later, don't throw the team under the bus by saying "Well I always said it was bad code, but I was outnumbered by the other team members."
Rejections are a natural part of the code review process.
The code review process isn't there to rubber stamp fixes from more senior people; it's there to protect and improve the quality of the code. There's nothing wrong with rejecting a fix provided you do it for the right reason, i.e. that the fix doesn't improve the code. If, after an open-minded review of the code, you still feel that the fix doesn't reduce the risk and/or magnitude of a demonstrable problem, then you should reject it. It's not personal, just your honest opinion. If the fixer disagrees, that's okay too, and at that point it becomes a problem for management to figure out. Just be sure to remain honest, open, and professional.
Responsibility cuts both ways.
You said that you don't want to be responsible for this change, apparently because you don't believe that there's a problem. However, you need to realize that if you're wrong and there is a problem, then you may end up being responsible for rejecting the code that would have avoided the problem.
Keeping a written log of the review process will help you keep your facts straight. Write down your thoughts and concerns while reviewing, description and results of any tests that you might run to measure the alleged problem and the fix, etc. If the issue escalates, you'll have a record of what you've done to support your position. If the matter comes up again in the future (it probably will if the fixer is attached to his own view), you'll have something to jog your memory.