I'm sorry you struggle.
Here's what stands out to me from my POV.
- he wants me to go and watch him have a BDSM scene with his acquaintance.
- he asked me to think it over a few days and answer
- he's willing to accept my decision.
- My first reaction was to say no, that it is outside my comfort zone and not something I want or need to be part of.
- my real objection is that I don't want to take (emotional?) responsibility for this other person and if he wants this it is up to him to negotiate it with C.
- I don't want the responsibility of being the one who everyone depends on (emotionally).
- (Unspoken -- I didn't expect to get close/involved with C, and then I did. I don't expect or want to get close with his play person so I rather not get involved there at all.)
I'm guessing you mean emotional responsibility there? I'm totally guessing the last bullet point. I could guess wrong.
But basically I'm hearing "NO."
You could tell him you don't want to attend his scene with the person, and that you expect him to also ask C about her feelings on it. Remind him you made it clear that C has to know and agree too, you are not willing to hide anything from her.
Seems straightforward enough. What is making it hard for you to tell him "no?"