1. Is opponent's draw offer unethical?
Offering a draw in positions that are much worse is frequently regarded as bad etiquette.
However, bad etiquette does not always equate to unethical behaviour.
In a tournament situation, it is correct to seek the best results in each pairing, and offering a draw in a worse position is a means to that end.
I assume that your opponent is not offering you a draw as a sign of disrespect or to annoy you (prohibited in most chess federations, e.g. USCF rule 20G).
Then the only unethical aspect of offering a draw as your opponent is that he is enticing unethical behaviour (as the tournament results are influenced, see next section), which cannot be ethical itself (according to Kant's categorical imperative / deontological ethics).
2. Is accepting the draw offer unethical?
As the answer to this question depends on the position (i.e. the chances of both players), I assume you have achieve[d] a completely winning position.
I interpret completely in the sense that you're not only winning by a large margin, but without major technical difficulties.
Let the probability that you get a worse result than a draw (i.e. you lose on the board or on time) be almost non-existent, and the probability that you win is very high.
Also, let there be no "justifying emergencies" (e.g. strong bodily/medical needs).
Then it is unethcial to offer or to accept a draw!
Your opponent has absolutely no right at all to expect you to accept his draw just because it doesn't make a difference for you regarding the tournament standings.
Even discussing the possible results with your opponent can be considered improper, as codified by USCF rule 20J.
His draw offer results from the pure self-interest of winning more prize money. However, this prize money belongs to someone else.
He has a losing position and does not deserve that half point.
When you accept the draw, you are depriving someone of the prize money they deserve for having played better than your opponent.
To see how unfair this is, imagine that your opponent was your friend and you agreed to a draw. Or even worse, you offer to accept his draw in exchange for some of his prize money. You would definitely need to expect facing serious complaints about manipulating game results, e.g. USCF rule 20L:
Collusion to fix or throw games, whether before or during the game, in order to manipulate prize money, title norms,
ratings, or for any other purpose is illegal and may result in severe sanctions, [...]
The fair final tournament standings result from a tournament, where each participant seeked to achieve the best result in every pairing.
You should play as if you were the player who is competing with your opponent for that prize money.
A minor side effect: if the tournament is rated, you are also harming your next opponent in a rated game, as your rating should increase in consequence of the win. For a win, he will gain less rating than he should, and for a loss he'd suffer more than due.
