GalaGirl and Magdlyn, you have both helped me a lot, thank you. It has made such a difference to be able to work through this.
I don't have a problem with saying no in general, but I do when I could prevent a potential problem. I think I am part of his problem as well as part of his solution because I would be so seriously angry with him if he messed up on something like this (as far as it involves another human being with needs, feelings and the potential to be hurt). I have given him a bit of a roasting once or twice (in the past) for being insensitive to Cs feelings, but I do try my best to keep out of things between them.
I think GalaGirl is right that I feel a bit 'meh' about it. If I could say that I would find it deeply upsetting to watch/be present it would be easier to say no. I wouldn't find it upsetting to be there, it is just something I don't want to be part of (in much the same way that I don't want to watch him cutting his toe nails). It just took me a while to give myself permission to say no for no other reason than I wanted to say no. If he goes ahead with this he has to be responsible enough to deal with all aspects of it responsibly. Magdlyn's words have given me 'permission' to be even more direct than I may have otherwise felt able to. Thank you both for you help and advice.