In answering this question, we first have to remember that Kant’s ethics say that specific acts are not morally commendable or condemnable in and of themselves. There is no such thing as an act that is morally good or bad. Instead, our motives make our actions good or bad. Therefore, in order to say whether assisted suicide is morally commendable, we would have to know why the person is helping someone else commit suicide. The person’s reasons for helping the other person die will determine whether the act is moral.
According to Kant, we have to use the categorical imperative to determine whether our actions are morally commendable. The categorical imperative states that one must
Act only on that maxim through which you can at the same time will that it should become a universal law [of nature].
In other words, we have to look at what rule we are using to justify a given action. We then have to ask if we would be willing to have that rule apply to everyone in our situation.
From this, we can see that assisted suicide could be morally good or morally bad. If I help my father die because I want a larger inheritance and I want it sooner, my maxim/rule is something like “Person A should help Person B die if Person B’s death will help Person A financially.” This is surely a terrible universal law as it gives anyone the right to help me kill myself just so they can get money. In such a case, assisted suicide is not morally commendable.
But now let us say that my father, who is of sound mind, wants to die because he is in terrible pain and his life is miserable. Let us further say that I will not benefit from his death in any way. Now, my maxim/rule is “Person A should help Person B die if Person B is suffering terribly and if Person B freely and competently decides that they want to die.” In this case, we can at least argue that assisted suicide is morally commendable. This maxim is one that some people could accept as a universal law of nature (though some will not).
When thinking about Kantian ethics, we must remember that acts are not morally good or bad. It is only our motives for actions that are commendable or blameworthy.