I honestly dont get why the Domme wouldnt allow him to use his safeword. That is just crazy and nowhere near SSC.
I never went that far with my current gf that she needed to use her safeword but then again I dont really intend to get there either.
If he needs to use a safeword the Dom/Domme/Master/Mistress failed imho. Yes I know it isnt really that simple but to take away his use of a safeword just sounds stupid to me.