I'm going to jump right into some real situations that occur with regularity for me. I don't feel my usual need to resort to hypotheticals, because most of the bases are covered by things that have actually happened and will continue to recur. As usual, I will tend to progress upwards in terms of the importance/severity of the situations. If we reach a point where you disagree with my thoughts or actions, you should stop and discuss THAT point with me. There is nothing to be gained by debating my conclusions if you disagree with one of my base premises.
There is a naked person in my bedroom. You knock on the door, and I invite you in. Seeing naked people is something that offends some people. Being seen while naked, by some large subset of potential viewers, offends some people. When I invite you in and you see a naked person, I have accepted responsibility for understanding both of your rules and have consented on both of your behalves. You are not repsonsible for invading the other person's privacy; if they are offended then I am responsible. They are not responsible for revealing themselves to you; I am. I know some people who would say that the naked person should cover up just in case the newcomer does not want to see nudity. I know a few people who would say the newcomer should always ask through the door for anyone inside the cover up. I say that neither person is responsible for taking either of those actions, because I have absolved them through my actions and decisions on their behalf.
In another situation, I am electrified, such that I give a shocking sensation to anyone I touch. You are someone I know, who has experienced the sensation before. You walk up to me with a friend of yours, who I haven't met before. You say to your friend "hold his hand". How should I proceed from here? Having encountered this specific situation some hundreds of times I have mostly established that if the new person seems eager then I will allow them to grab my hand immediately and if they seem wary then I will explain what is going to happen to them first. I have upset a few people by not informing them of what would happen when they touched me. I have also scared away some people by explaining it to them, who I later found out did enjoy the sensation and would have wanted to try it, sans explanation. Many people will, at this point, say that I am partially (or even mostly or completely) responsible for the potential violation of consent here. You, as their friend, have established by setting up the situation that YOU are willing to subject them to this sensation without their consent.
There is an alternate version of this scenario, still not hypothetical, where the shock is delivered by a device instead of by my hand, and the device is simply sitting on a table. When you say to your friend "touch that device", and they follow your instructions, the total responsibility for the outcome is split between you and them. If I have responsibility in the previous scenario, where did it come from? Do you or they become less responsible for the outcome because of my participation?
This concept extends to sex. Some people want anonymous sex, quite literally. I have been tasked, in the past, with arranging for a friend to be fucked by multiple people that I knew and she did not, without her meeting them beforehand or them being able to identify each other during. I am currently involved in a similar situaton, although more aimed at second base than at fucking. How could any of my friends accept that invitation, without getting consent from her? They trusted me to enforce and implement their requirements. How could she ask me to have them do this, without getting consent directly from them? She trusted me to convey and enforce her preferences and requirements. I, as the initator and facilitator, have willingly accepted the burden of implementing the consent of the other parties, and I accept responsibility for any errors in that regard.
Now, combining all of the above... You walk up to me with your friend, knowing that I am electrified, cover her eyes, pull down her top, and say to me "lick her nipple". You have chosen to offer third party consent on her behalf, for her to be topless in public. You have consented on behalf of the other people in the room, to be exposed to nudity. And you have consented on her behalf to be ignorant of my identity, to be touched sexually, and to experience an unexpected sensation. This is not a hypothetical, nor even an isolated incident.
So, given all of this information, where do you draw the line? Can you, should you, would you accept consent from one person on behalf of another, and if so then for what actions? Are there things about their relationship that would affect your decision, such as if you know they are acquaintances, friends, lovers, married, in a D/s relationship, kinky, sexually open, polyamorous, etc? Would you give that consent, on behalf of someone else? Would you let another person give consent for you? When, and why? Have you already told them that? Does it require more trust for you to accept consent from me for someone else than for you to accept consent from me for me?