It's literally ROLEPLAY, and Sarah herself already knew the role Sandra was going to be playing before hand, they might not have went into specifics or discussed boundaries before hand, but that's what a safe word is for, and for some people they don't like to discuss boundaries before hand, because it makes it a less authentic experience. Everyone is different of course, and some people have to approach things differently, but not once did Sarah make it seem like she was having a bad time during the actual sex itself. After she had her release, she wasn't able to handle it probably as well as she thought she would and got in her head about it and started crying, which you're way to hung up on. It's not Sandra's fault, she played the role she was there to play, they all had a great time, no safe word was used, but it proved to be a bit much for Sarah afterwards, that doesn't make Sandra a bitch for playing a role, and as soon as Sandra realized she may be pushing a bit hard once the MC took some control back, she didn't get upset about it, or try to take back control, she just continued to enjoy it like they had been.
This is the last thing I'll say on it, but yeah you're really making it seem like Sandra was in the wrong when she wasn't, hell she handled it well by being there for her afterwards, and going forward I'm sure all 3 of them will be a lot better for it.