The answer should always be, whether you're pro-choice suicide or not, is to talk that person out of it. This is not out of disrespect for the possibility that they are fully aware of what they are doing, but rather cautious covering-all-the-bases in case they are not. You can't take the risk that by agreeing with them on the sorry and disgusting state of the world you are enticing them to commit an act that they would otherwise regret. You have to take things cautiously, ask them why they do not want to exist, ask them how long they've been steady in this desire, and moreover, if there is anything that would change their desire. There, are probably hundreds more questions you could ask to ascertain whether a person is suicidal because life is currently hard and it looks like a good way to escape, or suicidal because life has always been hard and it seems the best choice in this circumstance.
There's a reason you can't just walk into a Euthanasia clinic and not come out the next day, or rather come out in a body bag the next day. If you really want to commit suicide, not out of acute pain, but chronic pain, or a hatred of life, then you should be prepared to wait at least a little longer. If you cannot do that, then it may be possible you are correct in your wish to die, but it's actually more likely that you would choose not to had you been in a better state of mind. These are the suicide-attempters that can truly be saved. You cannot save someone who TRULY wants to die, unless you save them from life in assisting their suicide, which is illegal in a lot of places.
So is it right to force more suffering onto people to make sure other people who are not in the right state of mind come to the right state of mind? It's a simple trade-off. How many lives wasted are you prepared to see for wishes fulfilled? I think most people would come to the conclusion that making the suicidal wait to commit suicide is a valid alternative to having people who would otherwise not be suicidal if they were thinking straight dying unnecessarily. Therefore, you should talk people out of suicide or at least ask them to wait for a while before supporting them in their wish to die.